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 ```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 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 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 #include #include #include #include   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 #include #include #include   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 #include #include #include #include #include   /* 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 [start]