midx: stop ignoring malformed oid fanout chunk

When we load the oid-fanout chunk, our callback checks that its size is
reasonable and returns an error if not. However, the caller only checks
our return value against CHUNK_NOT_FOUND, so we end up ignoring the
error completely! Using a too-small fanout table means we end up
accessing random memory for the fanout and segfault.

We can fix this by checking for any non-zero return value, rather than
just CHUNK_NOT_FOUND, and adjusting our error message to cover both
cases. We could handle each error code individually, but there's not
much point for such a rare case. The extra message produced in the
callback makes it clear what is going on.

The same pattern is used in the adjacent code. Those cases are actually
OK for now because they do not use a custom callback, so the only error
they can get is CHUNK_NOT_FOUND. But let's convert them, as this is an
accident waiting to happen (especially as we convert some of them away
from pair_chunk). The error messages are more verbose, but it should be
rare for a user to see these anyway.

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

1 files changed, 19 insertions, 1 deletions

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 1bcc02004d..b8fe85aeba 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -2,6 +2,7 @@
 
 test_description='multi-pack-indexes'
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-chunk.sh
 
 GIT_TEST_MULTI_PACK_INDEX=0
 objdir=.git/objects
@@ -438,7 +439,7 @@ test_expect_success 'verify extended chunk count' '
 
 test_expect_success 'verify missing required chunk' '
 	corrupt_midx_and_verify $MIDX_BYTE_CHUNK_ID "\01" $objdir \
-		"missing required"
+		"required pack-name chunk missing"
 '
 
 test_expect_success 'verify invalid chunk offset' '
@@ -1055,4 +1056,21 @@ test_expect_success 'repack with delta islands' '
 	)
 '
 
+corrupt_chunk () {
+	midx=.git/objects/pack/multi-pack-index &&
+	test_when_finished "rm -rf $midx" &&
+	git repack -ad --write-midx &&
+	corrupt_chunk_file $midx "$@"
+}
+
+test_expect_success 'reader notices too-small oid fanout chunk' '
+	corrupt_chunk OIDF clear 00000000 &&
+	test_must_fail git log 2>err &&
+	cat >expect <<-\EOF &&
+	error: multi-pack-index OID fanout is of the wrong size
+	fatal: multi-pack-index required OID fanout chunk missing or corrupted
+	EOF
+	test_cmp expect err
+'
+
 test_done