# How to recover an object from scratch

I was recently presented with a repository with a corrupted packfile, and was asked if the data was recoverable. This post-mortem describes the steps I took to investigate and fix the problem. I thought others might find the process interesting, and it might help somebody in the same situation.

Note: In this case, no good copy of the repository was available. For the much easier case where you can get the corrupted object from elsewhere, see this howto.

I started with an fsck, which found a problem with exactly one object (I’ve used $pack and $obj below to keep the output readable, and also because I’ll refer to them later):

        $ git fsck
        error: $pack SHA1 checksum mismatch
        error: index CRC mismatch for object $obj from $pack at offset 51653873
        error: inflate: data stream error (incorrect data check)
        error: cannot unpack $obj from $pack at offset 51653873

The pack checksum failing means a byte is munged somewhere, and it is presumably in the object mentioned (since both the index checksum and zlib were failing).

Reading the zlib source code, I found that "incorrect data check" means that the adler-32 checksum at the end of the zlib data did not match the inflated data. So stepping the data through zlib would not help, as it did not fail until the very end, when we realize the crc does not match. The problematic bytes could be anywhere in the object data.

The first thing I did was pull the broken data out of the packfile. I needed to know how big the object was, which I found out with:

        $ git show-index <$idx | cut -d' ' -f1 | sort -n | grep -A1 51653873
        51653873
        51664736

Show-index gives us the list of objects and their offsets. We throw away everything but the offsets, and then sort them so that our interesting offset (which we got from the fsck output above) is followed immediately by the offset of the next object. Now we know that the object data is 10863 bytes long, and we can grab it with:

      dd if=$pack of=object bs=1 skip=51653873 count=10863

I inspected a hexdump of the data, looking for any obvious bogosity (e.g., a 4K run of zeroes would be a good sign of filesystem corruption). But everything looked pretty reasonable.

Note that the "object" file isn’t fit for feeding straight to zlib; it has the git packed object header, which is variable-length. We want to strip that off so we can start playing with the zlib data directly. You can either work your way through it manually (the format is described in Documentation/technical/pack-format.txt), or you can walk through it in a debugger. I did the latter, creating a valid pack like:

        # pack magic and version
        printf 'PACK\0\0\0\2' >tmp.pack
        # pack has one object
        printf '\0\0\0\1' >>tmp.pack
        # now add our object data
        cat object >>tmp.pack
        # and then append the pack trailer
        /path/to/git.git/test-sha1 -b <tmp.pack >trailer
        cat trailer >>tmp.pack

and then running "git index-pack tmp.pack" in the debugger (stop at unpack_raw_entry). Doing this, I found that there were 3 bytes of header (and the header itself had a sane type and size). So I stripped those off with:

        dd if=object of=zlib bs=1 skip=3

I ran the result through zlib’s inflate using a custom C program. And while it did report the error, I did get the right number of output bytes (i.e., it matched git’s size header that we decoded above). But feeding the result back to "git hash-object" didn’t produce the same sha1. So there were some wrong bytes, but I didn’t know which. The file happened to be C source code, so I hoped I could notice something obviously wrong with it, but I didn’t. I even got it to compile!

I also tried comparing it to other versions of the same path in the repository, hoping that there would be some part of the diff that didn’t make sense. Unfortunately, this happened to be the only revision of this particular file in the repository, so I had nothing to compare against.

So I took a different approach. Working under the guess that the corruption was limited to a single byte, I wrote a program to munge each byte individually, and try inflating the result. Since the object was only 10K compressed, that worked out to about 2.5M attempts, which took a few minutes.

The program I used is here:

    #include <stdio.h>
    #include <unistd.h>
    #include <string.h>
    #include <signal.h>
    #include <zlib.h>

    static int try_zlib(unsigned char *buf, int len)
    {
            /* make this absurdly large so we don't have to loop */
            static unsigned char out[1024*1024];
            z_stream z;
            int ret;

            memset(&z, 0, sizeof(z));
            inflateInit(&z);

            z.next_in = buf;
            z.avail_in = len;
            z.next_out = out;
            z.avail_out = sizeof(out);

            ret = inflate(&z, 0);
            inflateEnd(&z);
            return ret >= 0;
    }

    /* eye candy */
    static int counter = 0;
    static void progress(int sig)
    {
            fprintf(stderr, "\r%d", counter);
            alarm(1);
    }

    int main(void)
    {
            /* oversized so we can read the whole buffer in */
            unsigned char buf[1024*1024];
            int len;
            unsigned i, j;

            signal(SIGALRM, progress);
            alarm(1);

            len = read(0, buf, sizeof(buf));
            for (i = 0; i < len; i++) {
                    unsigned char c = buf[i];
                    for (j = 0; j <= 0xff; j++) {
                            buf[i] = j;

                            counter++;
                            if (try_zlib(buf, len))
                                    printf("i=%d, j=%x\n", i, j);
                    }
                    buf[i] = c;
            }

            alarm(0);
            fprintf(stderr, "\n");
            return 0;
    }

I compiled and ran with:

      gcc -Wall -Werror -O3 munge.c -o munge -lz
      ./munge <zlib

There were a few false positives early on (if you write "no data" in the zlib header, zlib thinks it’s just fine :) ). But I got a hit about halfway through:

      i=5642, j=c7

I let it run to completion, and got a few more hits at the end (where it was munging the crc to match our broken data). So there was a good chance this middle hit was the source of the problem.

I confirmed by tweaking the byte in a hex editor, zlib inflating the result (no errors!), and then piping the output into "git hash-object", which reported the sha1 of the broken object. Success!

I fixed the packfile itself with:

      chmod +w $pack
      printf '\xc7' | dd of=$pack bs=1 seek=51659518 conv=notrunc
      chmod -w $pack

The `\xc7` comes from the replacement byte our "munge" program found. The offset 51659518 is derived by taking the original object offset (51653873), adding the replacement offset found by "munge" (5642), and then adding back in the 3 bytes of git header we stripped.

After that, "git fsck" ran clean.

As for the corruption itself, I was lucky that it was indeed a single byte. In fact, it turned out to be a single bit. The byte 0xc7 was corrupted to 0xc5. So presumably it was caused by faulty hardware, or a cosmic ray.

And the aborted attempt to look at the inflated output to see what was wrong? I could have looked forever and never found it. Here’s the diff between what the corrupted data inflates to, versus the real data:

      -       cp = strtok (arg, "+");
      +       cp = strtok (arg, ".");

It tweaked one byte and still ended up as valid, readable C that just happened to do something totally different! One takeaway is that on a less unlucky day, looking at the zlib output might have actually been helpful, as most random changes would actually break the C code.

But more importantly, git’s hashing and checksumming noticed a problem that easily could have gone undetected in another system. The result still compiled, but would have caused an interesting bug (that would have been blamed on some random commit).