• The primary answer is that it is not meaningful to compare them. Dramatically speaking, hfsdebug is the tip to the iceberg that is fileXray. It would be contrived to say that a bicycle is similar to a fighter jet because they both have wheels and can both be used to get from point A to point B. Similarly, it would not be useful to do a point-by-point differentiation between fileXray and hfsdebug even though they both “do things with HFS+ volumes.” hfsdebug’s functionality is a small, strict subset of fileXray’s functionality. Still, to list a few things for the sake of this post, fileXray has things like the following that hfsdebug does not:
  • Comprehensive forensics features
  • Ability to detect and parse GPT, APM, and MBR partition types
  • Large number (28 at the time of this writing) of built-in filters meant for power users, security folks, and forensics professionals
  • Built-in virtual file systems for performing advanced analyses
  • Checksumming of various types of volume data
  • Disk usage analytics
  • Volume content retrieval
  • Decompression of HFS+ compressed content
  • Support for fork-based extended attributes
  • Support for external journals
  • Ability to harvest recently used file system object names from the journal
  • Ability to trawl for volume content via arbitrarily extensible signature matching
  • Ability to scavenge for deleted content
  • Real-time file activity monitoring
  • Ability to reverse map bytes to files
• In terms of code-base sizes, hfsdebug is about 12,000 lines of source code, whereas fileXray is about 105,000 lines. (For those prone to misreading numbers, fileXray has a code-base that is nearly 9 times larger.)
• In case of the few things that both hfsdebug and fileXray can do, fileXray can be up to 20 times faster than hfsdebug. In other words, what could take up to several minutes with hfsdebug may be done by fileXray in just a few seconds.
• fileXray comes as a Universal Binary consisting of 64-bit Intel, 32-bit Intel, and 32-bit PowerPC versions, whereas hfsdebug is 32-bit PowerPC only and must be run under Rosetta on Intel Macs.
• fileXray works on Mac OS X versions 10.5 (Leopard) and newer, including 10.7 (Lion). hfsdebug works on Mac OS X versions 10.4 (Tiger) through 10.6 (Snow Leopard), assuming you have Rosetta installed. Not all features of hfsdebug work on all supported versions of Mac OS X, however.
• Some features of fileXray are simply too powerful and too cool to not be reiterated. For example, you must check out the Scavenger File System and the Arbitrary File System. The former lets you mount scavengable (deleted but recoverable) content as a volume, allowing you to access such content through applications of your choice! The latter lets you access arbitrary byte ranges on a volume through a convenient interface.
• fileXray is a current product, whereas hfsdebug has now been retired because of flaws that cause it to display incorrect results under certain circumstances.

To sum up, fileXray is not a better hfsdebug, but a different beast altogether.

Start with the ebook. If you are one of the target audiences, it will be worth your time.

  /* tiny.c */
  int main(void) { return 42; }

Apparently recent Linux kernels do stricter checks on ELF executables, because of which the aforementioned 45-byte executable no longer works. A slightly larger, 64-byte version still works at the time of this writing.

Anyway, as far as tiny executables go, ELF on Linux is taken care of. It would be interesting to repeat a similar experiment for Mach-O executables on Mac OS X.

Let us first see how large the executable generated from the C program is on Mac OS X.

$ cat tiny.c
main() { return 42; }
$ sw_vers
...
ProductVersion: 10.5.6
...
$ gcc -Oz -o tiny tiny.c
$ strip tiny
$ ls -las tiny
32 -rwxr-xr-x  1 singh  wheel  12348 Mar 15 17:26 tiny

The following assembly language program can be compiled to generate a 165-byte Mach-O executable that runs on Mac OS X and returns the wisely chosen value 42.

; tiny.asm for Mac OS X (Mach-O Object File Format)
; nasm -f bin -o tiny tiny.asm

BITS 32
        org   0x1000

        db    0xce, 0xfa, 0xed, 0xfe       ; magic
        dd    7                            ; cputype (CPU_TYPE_X86)
        dd    3                            ; cpusubtype (CPU_SUBTYPE_I386_ALL)
        dd    2                            ; filetype (MH_EXECUTE)
        dd    2                            ; ncmds
        dd    _start - _cmds               ; cmdsize
        dd    0                            ; flags
_cmds:
        dd    1                            ; cmd (LC_SEGMENT)
        dd    44                           ; cmdsize
        db    "__TEXT"                     ; segname
        db    0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ; segname
        dd    0x1000                       ; vmaddr
        dd    0x1000                       ; vmsize
        dd    0                            ; fileoff
        dd    filesize                     ; filesize
        dd    7                            ; maxprot

        dd    5                            ; cmd (LC_UNIXTHREAD)
        dd    80                           ; cmdsize
        dd    1                            ; flvaor (i386_THREAD_STATE)
        dd    16                           ; count (i386_THREAD_STATE_COUNT)
        dd    0, 0, 0, 0, 0, 0, 0, 0       ; state
        dd    0, 0, _start, 0, 0, 0, 0, 0  ; state
_start:
        xor   eax,eax
        inc   eax
	push  byte 42
        sub   esp, 4
        int   0x80                         ; _exit(42)

filesize equ  $ - $$

We can compile and run this program as follows, verifying that its return value is indeed 42.

$ nasm -f bin -o tiny tiny.asm
$ ls -las tiny
8 -rw-r--r--  1 singh  admin  165 Mar 15 12:21 tiny
$ chmod 755 tiny
$ ./tiny
$ echo $?
42

Some points to note:

• The executable—Mach-O header, load commands, the text segment—is manually crafted in assembly using the nasm 80x86 assembler. The C compiler toolchain is not involved.
• The executable is unusual for Mac OS X in that the dynamc link editor (dyld) is not involved in running it. No dynamic libraries are involved either.
• The program makes a "direct" system call through the int 0x80 interface. This is a big no-no on Mac OS X—production code should not be bypassing the C library for making system calls, but then hopefully you won't be writing production code using such techniques. A specific caveat is that system call implementation may do things differently—in terms of arguments, return values, and such—from the user-callable interface. The implementation may also change across system revisions, so such code may break with a system update.
• The int 0x80 system call path is a legacy path on Mac OS X. It may be removed from Mac OS X some day, in which case the program would need to be modified to use newer alternatives such as sysenter.
• The executable is not a "correct" Mach-O file, even though the kernel can parse and run it in our case. The reason it's incorrect is because Mach-O load commands have been deliberately made to overlap each other to save some bytes. The otool object-file introspection command will be quite unhappy with this executable. In the following output, inconsistencies are shown in red.

$ otool -l tiny
tiny:
Load command 0
      cmd LC_SEGMENT
  cmdsize 44 Inconsistent size
  segname __TEXT
   vmaddr 0x00001000
   vmsize 0x00001000
  fileoff 0
 filesize 165
  maxprot 0x00000007
 initprot 0x00000005
   nsects 80
    flags 0x1
Section
  sectname
   segname  (does not match segment)
      addr 0x00000000
      size 0x00000000
    offset 0
     align 2^4248 (16777216)
    reloff 0
    nreloc 0
     flags 0x00000000
 reserved1 0
 reserved2 0
section structure command extends past end of load commands
Section
  sectname
   segname  (does not match segment)
      addr 0x00000000
      size 0x00000000
    offset 0
     align 2^0 (1)
    reloff 0
    nreloc 0
     flags 0x00000000
 reserved1 0
 reserved2 0

At the cost of roughly 80 additional bytes, we can create a "more correct" Mach-O executable that otool will be happy with.

; nicertiny.asm for Mac OS X (Mach-O Object File Format)
; nasm -f bin -o nicertiny nicertiny.asm

BITS 32
        org   0x1000

        db    0xce, 0xfa, 0xed, 0xfe       ; magic
        dd    7                            ; cputype (CPU_TYPE_X86)
        dd    3                            ; cpusubtype (CPU_SUBTYPE_I386_ALL)
        dd    2                            ; filetype (MH_EXECUTE)
        dd    2                            ; ncmds
        dd    _start - _cmds               ; cmdsize
        dd    0                            ; flags
_cmds:
        dd    1                            ; cmd (LC_SEGMENT)
        dd    124                          ; cmdsize
        db    "__TEXT"                     ; segname
        db    0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ; segname
        dd    0x1000                       ; vmaddr
        dd    0x1000                       ; vmsize
        dd    0                            ; fileoff
        dd    filesize                     ; filesize
        dd    7                            ; maxprot
        dd    5                            ; initprot
        dd    1                            ; nsects
        dd    0                            ; flags
        db    "__text"                     ; sectname
        db    0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ; sectname
        db    "__TEXT"                     ; segname
        db    0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ; segname
        dd    _start                       ; addr
        dd    _end - _start                ; size;
        dd    _start - 0x1000              ; offset
        dd    2                            ; align
        dd    0                            ; reloff
        dd    0                            ; nreloc
        dd    0                            ; flags
        dd    0                            ; reserved1
        dd    0                            ; reserved2

        dd    5                            ; cmd (LC_UNIXTHREAD)
        dd    80                           ; cmdsize
        dd    1                            ; flavor (i386_THREAD_STATE)
        dd    16                           ; count (i386_THREAD_STATE_COUNT)
        dd    0, 0, 0, 0, 0, 0, 0, 0       ; state
        dd    0, 0, _start, 0, 0, 0, 0, 0  ; state
_start:
        xor   eax, eax
        inc   eax
        push  dword 42
        sub   esp, 4
        int   0x80                         ; _exit(42)
_end:
filesize equ  $ - $$

Let us compile and run this version, and see what otool has to say.

$ nasm -f bin -o nicertiny nicertiny.asm
$ ls -las nicertiny
8 -rw-r--r--  1 singh  admin  248 Mar 15 14:49 nicertiny
$ chmod 755 nicertiny
$ ./nicertiny
$ echo $?
42
$ otool -l nicertiny
nicertiny:
Load command 0
      cmd LC_SEGMENT
  cmdsize 124
  segname __TEXT
   vmaddr 0x00001000
   vmsize 0x00001000
  fileoff 0
 filesize 248
  maxprot 0x00000007
 initprot 0x00000005
   nsects 1
    flags 0x0
Section
  sectname __text
   segname __TEXT
      addr 0x000010e8
      size 0x00000010
    offset 232
     align 2^2 (4)
    reloff 0
    nreloc 0
     flags 0x00000000
 reserved1 0
 reserved2 0
Load command 1
        cmd LC_UNIXTHREAD
    cmdsize 80
     flavor i386_THREAD_STATE
      count i386_THREAD_STATE_COUNT
	    eax 0x00000000 ebx    0x00000000 ecx 0x00000000 edx 0x00000000
	    edi 0x00000000 esi    0x00000000 ebp 0x00000000 esp 0x00000000
	    ss  0x00000000 eflags 0x00000000 eip 0x000010e8 cs  0x00000000
	    ds  0x00000000 es     0x00000000 fs  0x00000000 gs  0x00000000

It would be a nice exercise for the reader to try to shrink tiny.asm and nicertiny.asm even further, while retaining the high-level behavior of the corresponding executables. There are plenty of zeros lurking in there.

Here is the source for cpuinfo_x86.

The following is an example of the program in action. You must compile it as a 32-bit program. Besides Mac OS X, the program should also work on Linux, FreeBSD, and perhaps some other operating systems.

$ gcc -march=i386 -m32 -o cpuinfo_x86 cpuinfo_x86.c
$ ./cpuinfo_x86
# Identification
Vendor                : GenuineIntel
Brand String          : Intel(R) Xeon(R) CPU           X5550  @ 2.67GHz
Model Number          : 26 (Nehalem)
Family Code           : 6
Extended Model        : 1
Extended Family       : 0
Stepping ID           : 5
Signature             : 67237

# Address Bits
Physical Addressing   : 40
Virtual Addressing    : 48

# Multi-Core Information
Logical Processors (Threads) per Physical Processor : 16
Cores per Physical Package                          : 8

# Caches
## L1 Instruction Cache
Size                  : 32K
Line Size             : 64B
Sharing               : shared between 2 processor threads
Sets                  : 128
Partitions            : 1
Associativity         : 4

## L1 Data Cache
Size                  : 32K
Line Size             : 64B
Sharing               : shared between 2 processor threads
Sets                  : 64
Partitions            : 1
Associativity         : 8

## L2 Unified Cache
Size                  : 256K
Line Size             : 64B
Sharing               : shared between 2 processor threads
Sets                  : 512
Partitions            : 1
Associativity         : 8

## L3 Unified Cache
Size                  : 8M
Line Size             : 64B
Sharing               : shared between 16 processor threads
Sets                  : 8192
Partitions            : 1
Associativity         : 16

# Translation Lookaside Buffers
Instruction TLBs      : 7 large, 0 small
Data TLBs             : 32 large, 64 small

# Features
ACPI                  : Thermal Monitor and Software Controlled Clock
APIC                  : On-Chip APIC Hardware
CLFSH                 : CLFLUSH Instruction
CMOV                  : Conditional Move Instruction
CX16                  : CMPXCHG16B Instruction
CX8                   : CMPXCHG8 Instruction
DCA                   : Direct Cache Access
DE                    : Debugging Extension
DS                    : Debug Store
DS-CPL                : CPL Qualified Debug Store
DTES64                : 64-Bit Debug Store
EST                   : Enhanced Intel SpeedStep Technology
FPU                   : Floating-Point Unit On-Chip
FXSR                  : FXSAVE and FXSTOR Instructions
HTT                   : HyperThreading
MCA                   : Machine-Check Architecture
MCE                   : Machine-Check Exception
MMX                   : MMX Technology
MONITOR               : MONITOR/MWAIT Instructions
MSR                   : Model Specific Registers
MTRR                  : Memory Type Range Registers
PAE                   : Physical Address Extension
PAT                   : Page Attribute Table
PBE                   : Pending Break Enable
PDCM                  : Perfmon and Debug Capability
PGE                   : Page Global Enable
POPCNT                : POPCNT Instruction
PSE                   : Page Size Extension
PSE-36                : 36-Bit Page Size Extension
SEP                   : Fast System Call
SS                    : Self-Snoop
SSE                   : Streaming SIMD Extensions
SSE2                  : Streaming SIMD Extensions 2
SSE3                  : Streaming SIMD Extensions 3
SSSE3                 : Supplemental Streaming SIMD Extensions 3
SSE4.1                : Streaming SIMD Extensions 4.1
SSE4.2                : Streaming SIMD Extensions 4.2
TM                    : Thermal Monitor
TM2                   : Thermal Monitor 2
TSC                   : Time Stamp Counter
VME                   : Virtual Mode Extension
VMX                   : Virtual Machine Extensions
xTPR                  : xTPR Update Control

# Extended Features
EM64T                 : Intel Extended Memory 64 Technology
XD                    : Execution Disable

Despite the detailed information cpuinfo_x86 shows, it is still does not retrieve all possible information you can get through CPUID. Moreover, because I wrote it for the x86 version of Mac OS X, the program assumes that you have a relatively recent x86 processor and will likely not behave too well on old processors. Writing an exhaustive and legacy-aware CPUID program is a more rigorous programming exercise.

You should now be able to build the AncientFS source tree on Mac OS X, Linux, and FreeBSD. You must have the FUSE implementation for your platform installed to build and use AncientFS. Then, it should take a single make command to build.

$ svn co http://macfuse.googlecode.com/svn/trunk/filesystems/unixfs
$ cd unixfs/ancientfs
$ make # GNU make required. Use gmake on FreeBSD.
...

If it doesn’t build out-of-the-box on your system, you might want to tweak platform-specific settings in unixfs/ancientfs/Makefile first. The default settings assume that fuse is installed under /usr on Linux and under /usr/local on FreeBSD.

Because libfuse wants to use sem_init(), you will need to ensure on FreeBSD that the POSIX semaphore implementation is either statically compiled into the kernel (not the default on recent FreeBSD systems) or the sem kernel module is loaded. See sem(4) for details.

I’m traveling and don’t have good/much access to Linux or FreeBSD machines. Therefore, I haven’t been able to test this much. I do know that it builds and mounts some things on at least Linux 2.6.18 (Ubuntu) and FreeBSD 7.1-BETA2.

Now that the talk’s video is available. I’ve written some notes on the new file systems themselves. There is a lot of new information and new code for those interested in file systems, Mac OS X, and operating systems in general. Much of it is academically and practically useful, and some could have significant potential down the road. You could also think of it as a new year gift to the Mac OS X and open-source communities.

This is a meta note on how you can go about discovering and understanding the what, how, and why of everything I’m talking about.

1. MacFUSE State of the Union Talk (2008) Video

If you were not at the talk, watching the video would provide useful context and should help greatly in understanding what follows.

2. AncientFS

Next, you could look at AncientFS, which I introduced, among other things, during the talk. AncientFS lets you mount ancient (and in some cases, current-day) "data containers" as regular volumes on Mac OS X. It supports the following formats.

• v1tap—DECtape tap tape archive; UNIX V1
• v2tap—DECtape tap tape archive; UNIX V2
• v3tap—DECtape tap tape archive; UNIX V3
• ntap—DECtape/magtape tap tape archive; 1970 epoch
• tp—DECtape/magtape tp tape archive
• itp—UNIX itp tape archive
• dtp—UNIX dtp tape archive
• dump—Incremental file system dump (512-byte blocks, V7/bsd)
• dump1kI—Incremental file system dump (1024-byte blocks, V7/bsd)
• dump-vn—Incremental file system dump (512-byte blocks, bsd-vn)
• dump1k-vn—Incremental file system dump (1024-byte blocks, bsd-vn)
• v1ar—Very old (0177555) archive (.a) from First Edition UNIX
• v2ar—Very old (0177555) archive (.a) from Second Edition UNIX
• v3ar—Very old (0177555) archive (.a) from Third Edition UNIX
• ar—Current (!<arch>\n), old (0177545), or very old (0177555) archive (.a); use (v1|v2|v3)ar for UNIX V1/V2/V3 archives
• bcpio—Binary cpio archive (old); may be byte-swapped
• cpio_odc—ASCII (odc) cpio archive
• cpio_newc—New ASCII (newc) cpio archive
• cpio_newcrc—New ASCII (newc) cpio archive with checksum
• tar—ustar, pre-POSIX ustar, or V7 tar archive
• v1—First Edition UNIX file system
• v2—Second Edition UNIX file system
• v3—Third Edition UNIX file system
• v4—Fourth Edition UNIX file system
• v5—Fifth Edition UNIX file system
• v6—Sixth Edition UNIX file system
• v7—Seventh Edition UNIX file system
• v10—Tenth Edition UNIX file system
• 32v—UNIX/32V file system
• bsd—BSD file system (V7-style with fixed-length file names; e.g. 2.9BSD or 4.0BSD)
• bsd-vn—BSD file system (pre fast-file-system “UFS” with variable-length file names; e.g. 2.11BSD for PDP-11)

To learn more about AncientFS and how to use it, please read the AncientFS article. Enjoy seeing ancient data seamlessly in the modern namespaces of Mac OS X!

3. The UnixFS Layer

AncientFS also led to UnixFS, a general-purpose abstraction layer that proved useful in getting several other "alien" file systems up and running on Mac OS X. It is particularly useful for "Unix-style" file systems, where you need concepts such as on-disk and in-memory superblocks and inodes. As I mentioned during the talk, I used UnixFS in conjunction with ad-hoc "Linux emulation" to bring support for the UFS, System V, and Minix file system families on Mac OS X. That amounts to a large number of new file systems, but it was easier than it sounds because the idea was to take existing Linux kernel implementations of these file system families and make them work in user space on Mac OS X!

UnixFS is currently rather "beta" and not a formal API by any means. Since it is a programming interface and doesn’t do anything by itself, it will be of interest only to developers at this point. In future, it may also evolve into a "LinuxFS" layer that could make it even easier and faster to systematically make Linux kernel-based file systems work on other platforms with very few code changes. For now, you can browse some bits of code.

You can, however, use the aforementioned UnixFS-based file system families: UFS, System V, and Minix. To do so, check out the relevant part of the MacFUSE source tree and compile one or more (or all) of the UnixFS-based file systems—it is quite straightforward: a single make in the filesystems/unixfs/ subdirectory in the MacFUSE source tree should build all of them. (You must have MacFUSE installed, of course.)

$ svn co http://macfuse.googlecode.com/svn/trunk/filesystems/unixfs
$ cd unixfs
$ ls -F
Makefile	common/		sysvfs/
ancientfs/	minixfs/	ufs/
$ make
...
$

4. The UFS Family

This is a user-space implementation (read-only) of the UFS file system family. Most of the UFS-specific code comes from the Linux kernel and is largely unchanged. Specific UFS flavors supported are as follows.

• old—the oldest UFS format
• sun—used in SunOS/Solaris
• sunx86—used in the x86 versions of SunOS/Solaris
• hp—used in HP-UX
• nextstep—used in NEXTSTEP
• nextstep-cd—used in NEXTSTEP CDROMs
• openstep—used in OPENSTEP
• 44bsd—used in FreeBSD, NetBSD, OpenBSD, and Mac OS X
• ufs2—used in FreeBSD 5.x

5. The System V Family

This is a user-space implementation (read-only) of the System V file system family. Most of the sysvfs-specific code comes from the Linux kernel and is largely unchanged. Specific sysvfs flavors supported are as follows.

• svr2—used in SVR2
• svr4—used in SVR4
• xenix—used in Xenix
• coherent—used in Coherent Unix

6. The Minix Family

This is a user-space implementation (read-only) of the Minix file system family. Most of the minixfs-specific code comes from the Linux kernel and is largely unchanged.

The following image summarizes the new file system capabilities Mac OS X gets as a result of these exercises.

7. The "One More Thing" Thing

Although it isn’t directly file system related, the last thing I demonstrated during the talk was the ability to run ancient Unix (PDP-11) binaries seamlessly on Mac OS X. PDP-11 aside, the techniques used to do so are generally useful for research and experimentation because unlike Linux, Mac OS X does not allow developers to extend the set of binary formats that can be "natively" executed by the kernel. The demo shows the Fifth Edition Unix kernel being compiled on Mac OS X using the original C compiler toolchain from a Fifth Edition disk image mounted using AncientFS. Additionally, there’s an "authentic" reproduction of the following ominous error message by running the original mv executable from Sixth Edition Unix.

	values of β will give rise to dom!

For more details, please watch the last section of the talk video. This is very preliminary work for which no source code or binaries are available yet.

When you specify a file system object to HFSDebug using a pathname, how HFSDebug will treat the pathname usually depends on other arguments, or the lack of other arguments.

Something to Read and Forget: The “Legacy” Mode

A typical invocation is quite simple: you simply give HFSDebug a path. If the path exists on an HFS+ volume, HFSDebug will use the underlying volume as the one to operate upon.

$ sudo hfsdebug /mach_kernel
...

In this case, HFSDebug will begin by doing a stat(2) call on the path. (As you will see shortly, this and the entire “legacy mode” is optional—you can make HFSDebug do even this part “from scratch” and not use stat(2) or other file system calls on the volume.) Since the goal is to make it possible to examine file system’s internal structure as opposed to what the user “sees” through layers of interfaces, it matters whether the object in question is a file hard link, directory hard link, symbolic link, etc. Specifically, we need to get at the link reference that the pathname represents—not the target it resolves to. For symbolic links, we can use lstat(2) to give us the node ID of the reference. For file and directory hard links, we have to use something else.

How do we know if something is a hard link? HFSDebug examines the st_nlink (link count) and st_mode fields of the resultant stat structure.

In the case of regular files, a link count of 2 or higher means the object is a “known” hard link.

In the case of directories, what “link count” means is quite context-sensitive on HFS+. The stat structure’s st_nlink for a directory normally represents the directory’s item count. The directory itself (the "." entry) and its parent (the ".." entry) together add 2 to the count. Thus, if you have a directory with, say, 4 files and 4 subdirectories in it, a stat(2) call would report an st_nlink value of 10. However, if the folder count bit is enabled on the mount, the meaning of st_nlink changes: it then represents a count of only the subdirectories. In the aforementioned example, the link count would be 6 instead of 10. The folder count bit is currently only enabled for case-sensitive (HFSX) volumes. Besides the stat structure’s st_nlink field, HFS+ can separately provide the children count and the real hard link count for directories. The getattrlist(2) call can be used to retrieve these: they are the ATTR_DIR_ENTRYCOUNT and ATTR_DIR_LINKCOUNT directory attributes, respectively. Once we do know a directory’s “real” hard link count, again, a value of 2 or higher means it is a “known” hard link.

By a “known” file or directory hard link, I mean that we know that it currently is a hard link. That means the object we are looking at is a link reference and the “visible” node ID isn’t that of the reference but is that of its target. (See my previous post for more information.) In this case, HFSDebug will retrieve the object’s parent folder’s node ID and use it in conjunction with the object’s name to do a “from scratch” lookup of the object’s node ID. (This is a traditional { parent_nodeid, name } ==> nodeid lookup.)

However, things are not simpler if the link count of the object is 1. That’s because it could have had a higher link count in the past and the other links were deleted. Again, as I described earlier, the on-disk object continues to be a reference to the real content that lives in a special hidden HFS+ folder. If that’s the case though, the hidden folder will have a file or a folder whose name is formed from the object’s visible node ID: iNode%d or dir_%d for files and folders, respectively. If no such file or folder exists, as HFSDebug can look up from scratch, the object is not a current or past hard link.

If this sounds unnecessarily complex, well, some of it is. Until version 4, HFSDebug did not have the ability to process complete pathnames from scratch. With version 4, you can do things like the following on both mounted and unmounted volumes alike.

$ sudo hfsdebug -d /dev/disk0s2 /mach_kernel
...

Simpler, Better

In the new mode, HFSDebug will no longer do a stat(2)/lstat(2) or involve the file system otherwise. Obviously, if it has to support unmounted volumes, that’s how it has to be. It will take the pathname and process it component-by-component, which is easier to conceptualize than the “legacy” mode I described above. (Well, to be fair to the legacy mode, it became uglier with the advent of directory hard links.)

The following are examples of how HFSDebug will handle things based on some of its arguments.

$ sudo hfsdebug /foo/bar/baz
# legacy mode; will use stat(2)/lstat(2) to kick things off
...
$ sudo hfsdebug -d /dev/diskN /foo/bar/baz
# new mode; volume can be mounted or unmounted
...
$ sudo hfsdebug -P /foo/bar/baz
# new mode; uses root volume, which is obviously mounted
...
$ sudo hfsdebug -d /dev/diskN -P /foo/bar/baz
# new mode; volume can be mounted or unmounted

Just remember that you can use the -P argument (note that it’s the capital P) to specify a path and HFSDebug will use the new mode on both mounted and unmounted volumes. The path must be absolute.

Some Rules

Now, if HFSDebug is not involving the file system at all, it better be able to handle arbitrarily convoluted pathnames. Using things like realpath(3) is not an option since realpath(3) would want to call stat(2)/lstat(2). Say, we could have a path like the following.

/foo////././bar///../baz/../blah/..//////.././dir/are/you/../crazy/

Besides, there could be components in the path that are symbolic or hard links. Symbolic links could point to targets that have equally crazy pathnames. It’s not just a matter of canonicalizing the dots and the slashes. We have several requirements as illustrated by the following examples. (Some of these are simply HFSDebug conventions.)

• We must ensure that all intermediate components resolve to directories. They can be actual directories, valid symbolic links to directories, or directory hard links. Remember that in the case of symbolic or hard links, the on-disk object will be a “file”—HFSDebug will need to resolve them from scratch too.
• If there is a “..”, we must not blindly go “up” one level: we must ensure that what we are going back from is a directory. realpath(3) actually doesn’t care about this: it will canonicalize /path/to/file.txt/../file.txt to /path/to/file.txt.
• Although we do want to resolve intermediate components that are links, we must not resolve the terminal component if it happens to be a hard link or a symbolic link. That’s because our goal is to look at what’s on disk for the given path. Besides, in the case of a link, the details shown by HFSDebug will include the full pathname to the link’s target. If we wanted further details on the target, we could run HFSDebug on it.
• If the path has a terminal slash, HFSDebug will ensure that the component is a directory. If it happens to be a directory hard link or a symbolic link, HFSDebug will resolve it in this case. Consider an example: suppose there is a symbolic link /tmp/somesymlink. The following is what HFSDebug will do depending on the arguments and what the link points to.

# somesymlink points to a file
#
$ sudo hfsdebug -P /tmp/somesymlink
... # will show details of the link itself
$ sudo hfsdebug -P /tmp/somesymlink/
... # will complain that the link target is not a directory

# somesymlink points to a directory
#
$ sudo hfsdebug -P /tmp/somesymlink
... # will show details of the link itself
$ sudo hfsdebug -P /tmp/somesymlink/
... # will show details of the directory somesymlink points to

# somesymlink points to a non-existent target
#
$ sudo hfsdebug -P /tmp/somesymlink
... # will show details of the link itself
$ sudo hfsdebug -P /tmp/somesymlink/
... # will complain that path /tmp/somesymlink/ was not found on the volume

HFSDebug has been an extremely useful tool for me. I’ve used it to help explain the workings of HFS+ in the Mac OS X Internals book, to understand occasional mysterious behavior in HFS+ volumes, to search for file system objects, to generate interesting file system statistics (top N largest files, top N fragmented files, resource forks vs data forks, contiguous free space, and so on), and to create interesting demos. (For example, to show HFS+ mechanisms such as Hot File Clustering and On-the-Fly Defragmentation at work.)

HFS+ is the preferred and default volume format on Mac OS X. Even with exciting new developments such as ZFS support in Mac OS X, I don’t expect HFS+ to become obsolete any time soon. Today’s Macintosh computers, iPods, iPhones, and AppleTV’s all use HFS+.

With most major releases of Mac OS X, HFS+ has gained new capabilities. Features such as metadata journaling, on-the-fly file defragmentation, hot file clustering, extended attributes, access control lists, hard link chains (tracking hard links), and directory hard links have come to HFS+ in recent years. There have been “news reports” of compression being an upcoming feature in HFS+.

The most interesting technical things about HFS+ are not its features, but how several of the newer features are implemented. With the goal of retaining backward compatibility, new features have often been retrofitted, or shoehorned, if you will, into HFS+. Knowing such implementation details evokes different reactions in different people, ranging from “That’s a nifty way to implement this!” to “Gross!” This is something you can decide for yourself with the help of HFSDebug, which can show you exactly how the file system works.

New Features in HFSDebug

Now, every time a new feature is added to HFS+, HFSDebug likely (but not always) needs to be updated, say, to recognize and parse a new type of on-disk object such as a directory hard link. I’m releasing a new version of HFSDebug that has the following improvements.

• Ability to show details of directory hard links.
• Ability to show details of hard link chains.
• New built-in filters: atime (find files by access time), dirhardlink (list directory hard links), hardlink (list file hard links), and sxid (list setuid/setgid files).
• Ability to do component-wise path lookup from scratch, allowing you to analyze individual file system objects by path even on unmounted HFS+ volume.
• Support for Snow Leopard.
• Numerous subtle improvements and some bugfixes.

Let us take the new HFSDebug features for a spin.

Hard Link Chains

Although support for file hard links has been there in HFS+ before Leopard, the new chaining feature in Leopard can keep track of hard link chains, which are doubly linked list of file IDs connecting hard links together. Hard links to a file on HFS+ are conceptually similar to those on Unix systems: They represent multiple directory entries referring to common file content. Implementation-wise, HFS+ hard links use a special hard-link file for each directory entry. The common file content is stored in a special file: the indirect-node file. All indirect-node files are stored in the private metadata folder, a special directory (/\xE2\x90\x80\xE2\x90\x80\xE2\x90\x80\xE2\x90\x80HFS+ Private Data) that’s both normally invisible to the user and has a name that’s “hard” to type. It’s much easier to understand this through HFSDebug.

We begin by creating a file called file1. Before we create a hard link to this file, we examine its details using HFSDebug. That way, we can tell if anything about the file changes after link creation.

$ mkdir /tmp/test/
$ cd /tmp/test
$ echo "This is file1" > file1
$ sudo hfsdebug file1
...
  path                 = Leopard HD:/private/tmp/test/file1
# Catalog File Record
  type                 = file
  file ID              = 1927091
  flags                = 0000000000000010
...
  # BSD Info
  ownerID              = 501 (singh)
  groupID              = 0 (wheel)
  adminFlags           = 00000000
  ownerFlags           = 00000000
  fileMode             = -rw-r--r--
  linkCount            = 1
  textEncoding         = 0
  attrBlocks           = 0
  # Finder Info
  fdType               = 0
  fdCreator            = 0
  fdFlags              = 0000000000000000
...

Let us now make a hard link to this file.

$ ln file1 file2

Let us see if anything has changed about file1 now that we made a hard link to it.

$ sudo hfsdebug file1
...
  path                 = Leopard HD:/private/tmp/test/file1
# Catalog File Record
  type                 = file (hard link)
  indirect node file   = Leopard HD:/%0000%0000%0000%0000HFS+ Private Data/iNode1927091
  file ID              = 1927094
  flags                = 0000000000100010
                       . File has a thread record in the catalog.
                       . File has hardlink chain.
...
  # BSD Info
  ownerID              = 1927095 (previous link ID)
  groupID              = 0 (next link ID)
  adminFlags           = 00000000
  ownerFlags           = 00000010
                       . UF_IMMUTABLE (file may not be changed)
  fileMode             = -r--r--r--
  iNodeNum             = 1927091 (link reference number)
  textEncoding         = 0
  attrBlocks           = 0
  # Finder Info
  fdType               = 0x686c6e6b (hlnk)
  fdCreator            = 0x6866732b (hfs+)
  fdFlags              = 0000000100000000
                       . kHasBeenInited
...
  # Data Fork
  logicalSize          = 0 bytes
  # Resource Fork
  logicalSize          = 0 bytes

We see that a lot has changed! The on-disk nature of file1 has completely transformed. The original content has actually “moved” to an indirect-node file. What was file1 before has been replaced with a new directory entry altogether: one that has a new file ID within the file system. The new directory entry is also a file, but with several special properties. Its “type” and “creator” (as stored in the Finder Info) are hlnk and hfs+, respectively. It has been marked immutable. It has no content in either its data fork or its resource fork. Moreover, the owner and group ID on-disk fields have been repurposed to act as the previous and next links, respectively, in the hard link chain. We see that the previous link ID is 1927095. Let us use HFSDebug to show us information for that ID.

$ sudo hfsdebug -c 1927095
...
  path                 = Leopard HD:/private/tmp/test/file2
# Catalog File Record
  type                 = file (hard link)
  indirect node file   = Leopard HD:/%0000%0000%0000%0000HFS+ Private Data/iNode1927091
  file ID              = 1927095
  flags                = 0000000000100010
                       . File has a thread record in the catalog.
                       . File has hardlink chain.
...
  # BSD Info
  ownerID              = 0 (previous link ID)
  groupID              = 1927094 (next link ID)
  adminFlags           = 00000000
  ownerFlags           = 00000010
                       . UF_IMMUTABLE (file may not be changed)
  fileMode             = -r--r--r--
  iNodeNum             = 1927091 (link reference number)
  textEncoding         = 0
  attrBlocks           = 0
  # Finder Info
  fdType               = 0x686c6e6b (hlnk)
  fdCreator            = 0x6866732b (hfs+)
  fdFlags              = 0000000100000000
                       . kHasBeenInited
...
  # Data Fork
  logicalSize          = 0 bytes
  # Resource Fork
  logicalSize          = 0 bytes

We see that ID 1927095 corresponds to the other reference we just created: file2. The properties of this reference are similar to those of the other reference file1. They do differ in their file IDs. (They are indeed two separate on-disk file system objects.) They also differ in their previous and next links in the hard link chain. (We can confirm that file1 and file2 are connected together.)

The file’s content is in the indirect node file, which also now is the on-disk object with the original file ID (1927091). Let us use HFSDebug to look at that file.

$ sudo hfsdebug -c 1927091
...
  path                 = Leopard HD:/%0000%0000%0000%0000HFS+ Private Data/iNode1927091
# Catalog File Record
  type                 = file
  file ID              = 1927095
  flags                = 0000000000100010
                       . File has a thread record in the catalog.
                       . File has hardlink chain.
  reserved1            = 1927095 (first link ID)
...
  # BSD Info
  ownerID              = 501 (singh)
  groupID              = 0 (wheel)
  adminFlags           = 00000000
  ownerFlags           = 00000000
...
  # Finder Info
  fdType               = 0x686c6e6b (hlnk)
  fdCreator            = 0x6866732b (hfs+)
  fdFlags              = 0000000100000000
                       . kHasBeenInited
...
  # Data Fork
  logicalSize          = 14 bytes
  totalBlocks          = 1
  fork temperature     = no HFC record in B-Tree
  clumpSize            = 0
  extents              =   startBlock   blockCount      % of file
                             0xbb04b7          0x1       100.00 %
                         1 allocation blocks in 1 extents total.
                         1.00 allocation blocks per extent on an average.
  # Resource Fork
  logicalSize          = 0 bytes

As we see, the indirect-node file acts as the container for several of the original file’s properties. In particular, it has the original file’s content, the owner ID, and the group ID. A reserved field (reserved1) even contains the ID of the head of the hard link chain.

Of course, these are implementation details. HFS+ will show you the expected hard link semantics when you look at these files through the usual file system interfaces.

$ ls -las file1 file2
1927091 8 -rw-r--r--  2 singh  wheel  14 Nov  3 21:55 file1
1927091 8 -rw-r--r--  2 singh  wheel  14 Nov  3 21:55 file2
$ cat file1 file2
This is file1
This is file1

We see that both file1 and file2 show up with identical metadata, including the same “inode” number as you would expect. They also “have” the same content.

Note that if you now delete one of the hard links, say, file2, things will not revert back to how they were to begin with. You will have file1 as the only hard-link file along with the indirect-node file.

Let us look at directory hard links next.

Directory Hard Links

It’s not straightforward to create a directory hard link on Mac OS X. Well, that shouldn’t be surprising: directory hard links aren’t meant for third party developers, let alone users. They are essentially an implementation detail needed to make the Time Machine feature of Leopard work. Since we are talking about implementation details here, we will have to create a directory hard link or two—for experimentation, of course.

Leopard at the time of this writing requires the following conditions to be met for a directory hard link’s creation to be allowed. In the following list, “source” refers to the existing directory that will be pointed at by the new directory hard link “destination” that’s being created.

• The file system must be journaled HFS+.
• The parent directories of the source and destination must be different.
• The source’s parent must not be the root directory.
• The destination must not be in the root directory.
• The destination must not be a descendent of the source.
• The destination must not have any ancestor that’s a directory hard link.

If you meet all these conditions, you could create a directory hard link on an HFS+ volume under Mac OS X 10.5 and above. It’s then a matter of writing a program that uses the link() system call.

/* dirlink.c */

#include <stdio.h>
#include <unistd.h>

int
main(int argc, char** argv)
{
    int ret = -1;
    if (argc == 3) {
        ret = link(argv[1], argv[2]);
        if (ret) {
            perror("link");
        }
    }
    return ret;
}

In our /tmp/test/ testing directory, we’ll create a directory dir1 and a subdirectory subdir. It’s in subdir that we’ll create a hard link dir2 to dir1. This is because dir1 and dir2 can’t have the same parent.

$ gcc -Wall -o dirlink dirlink.c
$ mkdir dir1
$ mkdir subdir

Before we create the directory hard link, let us use HFSDebug to peek at the current on-disk details of dir1.

$ sudo hfsdebug dir1
...
  path                 = Leopard HD:/private/tmp/test/dir1
# Catalog Folder Record
  type                 = folder
  folder ID            = 1927398
  flags                = 0000000000000000
  valence              = 0
...
  # BSD Info
  ownerID              = 501 (singh)
  groupID              = 0 (wheel)
  adminFlags           = 00000000
  ownerFlags           = 00000000
  fileMode             = drwxr-xr-x
  linkCount            = 1
  textEncoding         = 0
  attrBlocks           = 0
  # Finder Info
  frRect               = (top = 0, left = 0), (bottom = 0, right = 0)
  frFlags              = 0000000000000000
  frLocation           = (v = 0, h = 0)
  opaque               = 0
  # Opaque Finder Info
  scrollPosition       = (v = 0, h = 0)
  reserved1            = 0
  Opaque Finder Flags  = 0000000000000000
  reserved2            = 0
  putAwayFolderID      = 0

Let us create the link and confirm that our expectations of directory hard link semantics are met.

$ ./dirlink dir1 subdir/dir2
$ ls -lasdi dir1 subdir/dir2
1927398 0 drwxr-xr-x  2 singh  wheel  68 Nov  3 22:59 dir1
1927398 0 drwxr-xr-x  2 singh  wheel  68 Nov  3 22:59 subdir/dir2
$ echo Hello > dir1/file
$ cat subdir/dir2/file
Hello

Everything looks in order. Let us now use HFSDebug to see what actually happened inside the file system. We looked at dir1‘s on-disk details earlier. We can now see what changed after we created a directory hard link to dir1.

$ sudo hfsdebug dir1
...
  path                 = Leopard HD:/private/tmp/test/dir1
# Catalog File Record
  type                 = file (alias, directory hard link)
  indirect folder      = Leopard HD:/.HFS+ Private Directory Data%000d/dir_1927398
  file ID              = 1927407
  flags                = 0000000000100010
                       . File has a thread record in the catalog.
                       . File has hardlink chain.
...
  # BSD Info
  ownerID              = 1927408 (previous link ID)
  groupID              = 0 (next link ID)
  adminFlags           = 00000000
  ownerFlags           = 00000010
                       . UF_IMMUTABLE (file may not be changed)
  fileMode             = -r--r--r--
  iNodeNum             = 1927398 (link reference number)
  textEncoding         = 0
  attrBlocks           = 0
  # Finder Info
  fdType               = 0x66647270 (fdrp)
  fdCreator            = 0x4d414353 (MACS)
  fdFlags              = 1000000000000000
                       . kIsAlias
  fdLocation           = (v = 0, h = 0)
  opaque               = 0
  # Data Fork
  logicalSize          = 0 bytes
  # Resource Fork
  logicalSize          = 464 bytes
  totalBlocks          = 1
  fork temperature     = no HFC record in B-Tree
  clumpSize            = 0
  extents              =   startBlock   blockCount      % of file
                             0xbae746          0x1       100.00 %
                         1 allocation blocks in 1 extents total.
                         1.00 allocation blocks per extent on an average.

  rsrc contents        = (up to 464 bytes)
       00 00 01 00 00 00 01 9e 00 00 00 9e 00 00 00 32 00 00 00 00 00 00 00 00
                                                     2
...
       00 00 00 00 00 00 00 1c 00 32 00 00 61 6c 69 73 00 00 00 0a 00 00 ff ff
                                   2        a  l  i  s
       00 00 00 00 00 00 00 00

We see that dir1‘s transformation is more drastic than what we had observed in the case of file hard links. After we created a directory hard link to dir1, it’s no longer a directory inside the file system. In fact, the “real” directory (that is, the link target) has moved to a special folder (/.HFS+ Private Directory Data\xd), just as the link target of a file hard link had moved to a (different) special folder. Its name within the special folder is dir_1927398, where the number represents the original “inode” number of dir1. However, dir1 hasn’t been replaced by another directory that points to the link target—it has been replaced by a file, or specifically, an alias. (Backward compatibility!) The immutable alias file has fdrp and MACS as its type and creator codes, respectively. It also as a resource fork. Moreover, we see that as in the case of file hard links, there exists a hard link chain.

Let us also examine the link target using HFSDebug. The path would be “hard” to type because of the characters in the special folder’s name. We can use the folder ID instead, which would be the original ID of dir1.

$ sudo hfsdebug -c 1927398
...
  path                 = Leopard HD:/.HFS+ Private Directory Data%000d/dir_1928557
# Catalog Folder Record
  type                 = folder
  folder ID            = 1927398
  flags                = 0000000000100100
                       . Folder has extended attributes.
                       . Folder has hardlink chain.
  valence              = 0
...
  # BSD Info
  ownerID              = 501 (singh)
  groupID              = 0 (wheel)
  adminFlags           = 00000000
  ownerFlags           = 00000000
  fileMode             = drwxr-xr-x
  linkCount            = 2
  textEncoding         = 0
  attrBlocks           = 0
  # Finder Info
  frRect               = (top = 0, left = 0), (bottom = 0, right = 0)
  frFlags              = 0000000000000000
  frLocation           = (v = 0, h = 0)
  opaque               = 0
...
# Attributes
...
  # Attribute Key
  keyLength            = 72
  pad                  = 0
  fileID               = 1927398
  startBlock           = 0
  attrNameLen          = 30
  attrName             = com.apple.system.hfs.firstlink
  # Inline Data
  recordType           = 0x10
  reserved[0]          = 0
  reserved[1]          = 0
  attrSize             = 8 bytes
  attrData             = 31 39 32 37 34 30 38 00
                          1  9  2  7  4  0  8     

We see mostly what we would expect given our previous observation of the implementation details of file hard links. There is one more thing in this case though: the folder has an extended attribute whose name is com.apple.system.hfs.firstlink and whose value is an encoding of the “inode” number of the head of the directory hard link chain.

HFSDebug Filters

At this point, you could use the built-in dirhardlink filter in HFSDebug to enumerate all directory hard links on the volume.

$ sudo hfsdebug --filter=builtin:dirhardlink
2 links -> dir_1927398
Leopard HD:/private/tmp/test/dir1 -> dir_1927398
Leopard HD:/private/tmp/test/subdir/dir2 -> dir_1927398

The filter prints both link targets and link references. For a link target, the number of references to it is printed before it. For a link reference, the target that it points to is printed after it.

By the way, filters are a very useful recent addition to HFSDebug. A fundamental capability of HFSDebug is to go over all the entries in the HFS+ catalog file. It uses this capability to generate many types of statistics. The recently added filter support makes it possible for you to write a program that plugs into HFSDebug and receives a callback for each catalog file entry. That way, you can examine each entry, apply arbitrary criteria, and show (or not show) details about that entry. Say, you wish to list all setuid/setgid files on an HFS+ volume. Sure, you could run a find command to do that. On one of my HFS+ volumes with about a million files and 200K folders, find takes a while to do this.

$ time sudo find / -xdev -type f \( -perm -4000 -o -perm -2000 \)
...
6.41s user 94.12s system 34% cpu 4:53.35 total
$

You could do this much faster with the sxid built-in HFSDebug filter, whose implementation is a mere ten lines of C code. (Of course, the absolute time taken will also depend on the underlying hardware, but we are only interested in the relative time difference.)

$ time sudo hfsdebug --filter=builtin:sxid
...
2.86s user 9.33s system 17% cpu 1:08.04 total
$

Note that many types of searches on HFS+ can also be done through the searchfs() system call, although it can be quite cumbersome to use. Of course, searchfs() cannot be done on an unmounted volume.

Specifying File System Objects by Path on Unmounted Volumes

As we have seen, a common use for HFSDebug is to have it display implementation details of individual file system objects. You could specify the object of interest in several ways: by providing its catalog node ID (CNID), by providing an “fsspec” style pair consisting of the parent folder’s CNID and the object’s name, or by providing a POSIX-style path to the object. The latter is often the easiest and most convenient to specify. However, until now, HFSDebug did not do component-wise path lookups itself—it used the operating system to convert the path to an inode number. This results in a few caveats. To begin with, it’s against the HFSDebug philosophy of not relying on the operating system for any HFS+-related operations. It also means that if the volume in question is not mounted (say, it’s corrupt and can’t be mounted or you are investigating something and don’t want to mount it), you can’t use paths to look at individual objects. You will have to dump all objects on the file system and then find the node ID of the object of interest. Moreover, even on a mounted volume, the operating system disallows path-based access to several files. (See Chapter 12 of Mac OS X Internals.) In such cases, again you will need to know the node ID of the object of interest, even on a mounted volume.

I’ve “fixed this issue” (or “added the feature”, depending on how you look at it) in the new version of HFSDebug. Say, if you have an unmounted volume on /dev/disk5s1 and you want to examine /tmp/foo/bar on it. Now you can simply do:

$ sudo hfsdebug -d /dev/disk5s1 /tmp/foo/bar
...

The semantics of symbolic link resolution are as follows. If the object (bar in this example) is a symbolic link itself, then HFSDebug will show you properties of bar and not what it points to. This is in line with HFSDebug philosophy and also how things work today on mounted volumes. If, however, a nonterminal component of the path is a symbolic link, HFSDebug will resolve it. Again, this is desirable.

That’s about it.

One More Thing

I can’t talk about HFSDebug’s Snow Leopard-specific features since the latter is under NDA. If you do have access to the latest Snow Leopard seed, try HFSDebug on it. For example, examine some standard Mac OS X files using HFSDebug.

$ sudo hfsdebug /bin/ls
...
$ sudo hfsdebug /etc/asl.conf
...
$ sudo hfsdebug /Applications/Mail.app/Contents/PkgInfo
...