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.
+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
-------
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
+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
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;
+}
+--------------------------