summaryrefslogtreecommitdiff
path: root/Documentation/howto/recover-corrupted-object-harder.txt
blob: 9c4cd0915fe3f3e14879184f3a474e559130bdef (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
Date: Wed, 16 Oct 2013 04:34:01 -0400
From: Jeff King <peff@peff.net>
Subject: pack corruption post-mortem
Abstract: Recovering a corrupted object when no good copy is available.
Content-type: text/asciidoc
 
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 link:recover-corrupted-blob-object.html[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
link:../technical/pack-format.html[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).
 
 
The adventure continues...
--------------------------
 
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).
 
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 `zlib` file. I then used the `inflate` 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 `sha1-munge`
program below to brute-force the correct byte.
 
Here's the inflate program (it's essentially `gunzip` but without the
`.gz` header processing):
 
--------------------------
#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;
}
--------------------------
 
And here is the `sha1-munge` program:
 
--------------------------
#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;
}
--------------------------