[PATCH v4 1/1] midx.c: fix an integer overflow

[Damien Robert <damien.olivier.robert@xxxxxxxxx>]

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 to check that `git multi-pack-index write` does
not write an midx when there is no objects, and another to check
that `git multi-pack-index verify` warns when it verifies an midx with no
objects. For this last test, use t5319/no-objects.midx which was
generated by an older version of git.

Signed-off-by: Damien Robert <damien.olivier.robert+git@xxxxxxxxx>
---
Following the recommandations I uploaded an empty midx to check we don't
regress.

 midx.c                      |  15 +++++++++++++++
 t/t5319-multi-pack-index.sh |  13 +++++++++----
 t/t5319/no-objects.midx     | Bin 0 -> 1116 bytes
 3 files changed, 24 insertions(+), 4 deletions(-)
 create mode 100644 t/t5319/no-objects.midx

diff --git a/midx.c b/midx.c
index 1527e464a7..a520e26395 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 +1130,15 @@ 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..10c35d445d 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -42,10 +42,15 @@ 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
+'
+
+test_expect_success "Warn if a midx contains no oid" '
+	cp "$TEST_DIRECTORY"/t5319/no-objects.midx .git/objects/pack/multi-pack-index &&
+	test_must_fail git multi-pack-index verify &&
+	rm .git/objects/pack/multi-pack-index
 '
 
 generate_objects () {
diff --git a/t/t5319/no-objects.midx b/t/t5319/no-objects.midx
new file mode 100644
index 0000000000000000000000000000000000000000..e466b8e08654c29effb5248cb109d81cfbcfd2f4
GIT binary patch
literal 1116
zcmebEbctYOWMKe-06#}xFoS`?!{5`z4T<doVY7Jn`@2EKSv;WfKnj_S5FKTWhQMeD
djEoT2!pSZW+;QcELdu7jD{8h)6W8x`1prin5MuxU

literal 0
HcmV?d00001

-- 
Patched on top of v2.26.0-51-ga7d14a4428 (git version 2.25.2)