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