964 lines
46 KiB
HTML
964 lines
46 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<!--[if IE]><meta http-equiv="X-UA-Compatible" content="IE=edge"><![endif]-->
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<meta name="generator" content="Asciidoctor 1.5.8">
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<title>How to recover an object from scratch</title>
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<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700">
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<style>
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/* Asciidoctor default stylesheet | MIT License | http://asciidoctor.org */
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/* Uncomment @import statement below to use as custom stylesheet */
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|
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</style>
|
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</head>
|
|
<body class="article">
|
|
<div id="header">
|
|
<h1>How to recover an object from scratch</h1>
|
|
</div>
|
|
<div id="content">
|
|
<div id="preamble">
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="sidebarblock">
|
|
<div class="content">
|
|
<div class="paragraph">
|
|
<p>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 <a href="recover-corrupted-blob-object.html">this howto</a>.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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):</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> $ 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</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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).</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> $ git show-index <$idx | cut -d' ' -f1 | sort -n | grep -A1 51653873
|
|
51653873
|
|
51664736</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Show-index gives us the list of objects and their offsets. We throw away
|
|
everything but the offsets, and then sort them so that our interesting
|
|
offset (which we got from the fsck output above) is followed immediately
|
|
by the offset of the next object. Now we know that the object data is
|
|
10863 bytes long, and we can grab it with:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> dd if=$pack of=object bs=1 skip=51653873 count=10863</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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
|
|
<a href="../technical/pack-format.html">Documentation/technical/pack-format.txt</a>),
|
|
or you can walk through it in a debugger. I did the latter, creating a
|
|
valid pack like:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> # 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/t/helper/test-tool sha1 -b <tmp.pack >trailer
|
|
cat trailer >>tmp.pack</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> dd if=object of=zlib bs=1 skip=3</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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!</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The program I used is here:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre>#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;
|
|
}</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>I compiled and ran with:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> gcc -Wall -Werror -O3 munge.c -o munge -lz
|
|
./munge <zlib</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> i=5642, j=c7</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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!</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>I fixed the packfile itself with:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> chmod +w $pack
|
|
printf '\xc7' | dd of=$pack bs=1 seek=51659518 conv=notrunc
|
|
chmod -w $pack</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The <code>\xc7</code> 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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>After that, "git fsck" ran clean.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre> - cp = strtok (arg, "+");
|
|
+ cp = strtok (arg, ".");</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>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).</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="_the_adventure_continues">The adventure continues…​</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>I ended up doing this again! Same entity, new hardware. The assumption
|
|
at this point is that the old disk corrupted the packfile, and then the
|
|
corruption was migrated to the new hardware (because it was done by
|
|
rsync or similar, and no fsck was done at the time of migration).</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This time, the affected blob was over 20 megabytes, which was far too
|
|
large to do a brute-force on. I followed the instructions above to
|
|
create the <code>zlib</code> file. I then used the <code>inflate</code> program below to pull
|
|
the corrupted data from that. Examining that output gave me a hint about
|
|
where in the file the corruption was. But now I was working with the
|
|
file itself, not the zlib contents. So knowing the sha1 of the object
|
|
and the approximate area of the corruption, I used the <code>sha1-munge</code>
|
|
program below to brute-force the correct byte.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Here’s the inflate program (it’s essentially <code>gunzip</code> but without the
|
|
<code>.gz</code> header processing):</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre>#include <stdio.h>
|
|
#include <string.h>
|
|
#include <zlib.h>
|
|
#include <stdlib.h>
|
|
|
|
int main(int argc, char **argv)
|
|
{
|
|
/*
|
|
* oversized so we can read the whole buffer in;
|
|
* this could actually be switched to streaming
|
|
* to avoid any memory limitations
|
|
*/
|
|
static unsigned char buf[25 * 1024 * 1024];
|
|
static unsigned char out[25 * 1024 * 1024];
|
|
int len;
|
|
z_stream z;
|
|
int ret;
|
|
|
|
len = read(0, buf, sizeof(buf));
|
|
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);
|
|
if (ret != Z_OK && ret != Z_STREAM_END)
|
|
fprintf(stderr, "initial inflate failed (%d)\n", ret);
|
|
|
|
fprintf(stderr, "outputting %lu bytes", z.total_out);
|
|
fwrite(out, 1, z.total_out, stdout);
|
|
return 0;
|
|
}</pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>And here is the <code>sha1-munge</code> program:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre>#include <stdio.h>
|
|
#include <unistd.h>
|
|
#include <string.h>
|
|
#include <signal.h>
|
|
#include <openssl/sha.h>
|
|
#include <stdlib.h>
|
|
|
|
/* eye candy */
|
|
static int counter = 0;
|
|
static void progress(int sig)
|
|
{
|
|
fprintf(stderr, "\r%d", counter);
|
|
alarm(1);
|
|
}
|
|
|
|
static const signed char hexval_table[256] = {
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 00-07 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 08-0f */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 10-17 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 18-1f */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 20-27 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 28-2f */
|
|
0, 1, 2, 3, 4, 5, 6, 7, /* 30-37 */
|
|
8, 9, -1, -1, -1, -1, -1, -1, /* 38-3f */
|
|
-1, 10, 11, 12, 13, 14, 15, -1, /* 40-47 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 48-4f */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 50-57 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 58-5f */
|
|
-1, 10, 11, 12, 13, 14, 15, -1, /* 60-67 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 68-67 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 70-77 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 78-7f */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 80-87 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 88-8f */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 90-97 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 98-9f */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* a0-a7 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* a8-af */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* b0-b7 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* b8-bf */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* c0-c7 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* c8-cf */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* d0-d7 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* d8-df */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* e0-e7 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* e8-ef */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* f0-f7 */
|
|
-1, -1, -1, -1, -1, -1, -1, -1, /* f8-ff */
|
|
};
|
|
|
|
static inline unsigned int hexval(unsigned char c)
|
|
{
|
|
return hexval_table[c];
|
|
}
|
|
|
|
static int get_sha1_hex(const char *hex, unsigned char *sha1)
|
|
{
|
|
int i;
|
|
for (i = 0; i < 20; i++) {
|
|
unsigned int val;
|
|
/*
|
|
* hex[1]=='\0' is caught when val is checked below,
|
|
* but if hex[0] is NUL we have to avoid reading
|
|
* past the end of the string:
|
|
*/
|
|
if (!hex[0])
|
|
return -1;
|
|
val = (hexval(hex[0]) << 4) | hexval(hex[1]);
|
|
if (val & ~0xff)
|
|
return -1;
|
|
*sha1++ = val;
|
|
hex += 2;
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
int main(int argc, char **argv)
|
|
{
|
|
/* oversized so we can read the whole buffer in */
|
|
static unsigned char buf[25 * 1024 * 1024];
|
|
char header[32];
|
|
int header_len;
|
|
unsigned char have[20], want[20];
|
|
int start, len;
|
|
SHA_CTX orig;
|
|
unsigned i, j;
|
|
|
|
if (!argv[1] || get_sha1_hex(argv[1], want)) {
|
|
fprintf(stderr, "usage: sha1-munge <sha1> [start] <file.in\n");
|
|
return 1;
|
|
}
|
|
|
|
if (argv[2])
|
|
start = atoi(argv[2]);
|
|
else
|
|
start = 0;
|
|
|
|
len = read(0, buf, sizeof(buf));
|
|
header_len = sprintf(header, "blob %d", len) + 1;
|
|
fprintf(stderr, "using header: %s\n", header);
|
|
|
|
/*
|
|
* We keep a running sha1 so that if you are munging
|
|
* near the end of the file, we do not have to re-sha1
|
|
* the unchanged earlier bytes
|
|
*/
|
|
SHA1_Init(&orig);
|
|
SHA1_Update(&orig, header, header_len);
|
|
if (start)
|
|
SHA1_Update(&orig, buf, start);
|
|
|
|
signal(SIGALRM, progress);
|
|
alarm(1);
|
|
|
|
for (i = start; i < len; i++) {
|
|
unsigned char c;
|
|
SHA_CTX x;
|
|
|
|
#if 0
|
|
/*
|
|
* deletion -- this would not actually work in practice,
|
|
* I think, because we've already committed to a
|
|
* particular size in the header. Ditto for addition
|
|
* below. In those cases, you'd have to do the whole
|
|
* sha1 from scratch, or possibly keep three running
|
|
* "orig" sha1 computations going.
|
|
*/
|
|
memcpy(&x, &orig, sizeof(x));
|
|
SHA1_Update(&x, buf + i + 1, len - i - 1);
|
|
SHA1_Final(have, &x);
|
|
if (!memcmp(have, want, 20))
|
|
printf("i=%d, deletion\n", i);
|
|
#endif
|
|
|
|
/*
|
|
* replacement -- note that this tries each of the 256
|
|
* possible bytes. If you suspect a single-bit flip,
|
|
* it would be much shorter to just try the 8
|
|
* bit-flipped variants.
|
|
*/
|
|
c = buf[i];
|
|
for (j = 0; j <= 0xff; j++) {
|
|
buf[i] = j;
|
|
|
|
memcpy(&x, &orig, sizeof(x));
|
|
SHA1_Update(&x, buf + i, len - i);
|
|
SHA1_Final(have, &x);
|
|
if (!memcmp(have, want, 20))
|
|
printf("i=%d, j=%02x\n", i, j);
|
|
}
|
|
buf[i] = c;
|
|
|
|
#if 0
|
|
/* addition */
|
|
for (j = 0; j <= 0xff; j++) {
|
|
unsigned char extra = j;
|
|
memcpy(&x, &orig, sizeof(x));
|
|
SHA1_Update(&x, &extra, 1);
|
|
SHA1_Update(&x, buf + i, len - i);
|
|
SHA1_Final(have, &x);
|
|
if (!memcmp(have, want, 20))
|
|
printf("i=%d, addition=%02x", i, j);
|
|
}
|
|
#endif
|
|
|
|
SHA1_Update(&orig, buf + i, 1);
|
|
counter++;
|
|
}
|
|
|
|
alarm(0);
|
|
fprintf(stderr, "\r%d\n", counter);
|
|
return 0;
|
|
}</pre>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div id="footer">
|
|
<div id="footer-text">
|
|
Last updated 2019-02-26 19:31:11 UTC
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|