use_pack: handle signed off_t overflow

A v2 pack index file can specify an offset within a packfile
of up to 2^64-1 bytes. On a system with a signed 64-bit
off_t, we can represent only up to 2^63-1. This means that a
corrupted .idx file can end up with a negative offset in the
pack code. Our bounds-checking use_pack function looks for
too-large offsets, but not for ones that have wrapped around
to negative. Let's do so, which fixes an out-of-bounds
access demonstrated in t5313.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -1041,6 +1041,8 @@ unsigned char *use_pack(struct packed_git *p,
 		die("packfile %s cannot be accessed", p->pack_name);
 	if (offset > (p->pack_size - 20))
 		die("offset beyond end of packfile (truncated pack?)");
+	if (offset < 0)
+		die("offset before end of packfile (broken .idx?)");
 
 	if (!win || !in_window(win, offset)) {
 		if (win)

t/t5313-pack-bounds-checks.sh

@@ -136,7 +136,7 @@ test_expect_success 'bogus offset into v2 extended table' '
 	test_must_fail git index-pack --verify $pack
 '
 
-test_expect_failure 'bogus offset inside v2 extended table' '
+test_expect_success 'bogus offset inside v2 extended table' '
 	# We need two objects here, so we can plausibly require
 	# an extended table (if the first object were larger than 2^31).
 	do_pack "$object $(git rev-parse HEAD)" --index-version=2 &&