[PATCH v3 1/1] midx.c: fix an integer overflow

[Damien Robert <damien.olivier.robert@gmail.com>]

When verifying a midx index with 0 objects, the
    m->num_objects - 1
overflows to 4294967295.

Fix this both by checking that the midx contains at least one oid,
and also that we don't write any midx when there is no packfiles.

Update the tests so that we check that `git multi-pack-index write` does
not write an midx when there is no object.

Signed-off-by: Damien Robert <damien.olivier.robert+git@gmail.com>
---
Since I did not receive any guidelines, I did not upload an midx with no
object to check in the tests. I just modified the current tests to check
that we don't produce an midx if there is no objects.

 midx.c                      | 13 +++++++++++++
 t/t5319-multi-pack-index.sh |  7 +++----
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/midx.c b/midx.c
index 1527e464a7..018acc7e76 100644
--- a/midx.c
+++ b/midx.c
@@ -923,6 +923,12 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	cur_chunk = 0;
 	num_chunks = large_offsets_needed ? 5 : 4;
 
+	if (packs.nr - dropped_packs == 0) {
+		error(_("no pack files to index."));
+		result = 1;
+		goto cleanup;
+	}
+
 	written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);
 
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
@@ -1124,6 +1130,13 @@ int verify_midx_file(struct repository *r, const char *object_dir, unsigned flag
 				    i, oid_fanout1, oid_fanout2, i + 1);
 	}
 
+	if (m->num_objects == 0) {
+		midx_report(_("the midx contains no oid"));
+		// remaining tests assume that we have objects, so we can
+		// return here
+		return verify_midx_error;
+	}
+
 	if (flags & MIDX_PROGRESS)
 		progress = start_sparse_progress(_("Verifying OID order in multi-pack-index"),
 						 m->num_objects - 1);
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 43a7a66c9d..d90dfce268 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -42,10 +42,9 @@ test_expect_success 'setup' '
 	EOF
 '
 
-test_expect_success 'write midx with no packs' '
-	test_when_finished rm -f pack/multi-pack-index &&
-	git multi-pack-index --object-dir=. write &&
-	midx_read_expect 0 0 4 .
+test_expect_success "don't write midx with no packs" '
+	test_must_fail git multi-pack-index --object-dir=. write &&
+	test_path_is_missing pack/multi-pack-index
 '
 
 generate_objects () {
-- 
Patched on top of v2.26.0 (git version 2.25.1)