[PATCH v2 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.

Signed-off-by: Damien Robert <damien.olivier.robert+git@gmail.com>
---
Should I add a test? It is a bit troublesome to generate a zero object midx
file since this patch prevents it from using 'midx write'...

 midx.c | 35 +++++++++++++++++++++++------------
 1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/midx.c b/midx.c
index 1527e464a7..2cece7f9ea 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;
@@ -1124,22 +1130,27 @@ int verify_midx_file(struct repository *r, const char *object_dir, unsigned flag
 				    i, oid_fanout1, oid_fanout2, i + 1);
 	}
 
-	if (flags & MIDX_PROGRESS)
-		progress = start_sparse_progress(_("Verifying OID order in multi-pack-index"),
-						 m->num_objects - 1);
-	for (i = 0; i < m->num_objects - 1; i++) {
-		struct object_id oid1, oid2;
+	if (m->num_objects == 0)
+		midx_report(_("Warning: the midx contains no oid."));
+	else
+	{
+		if (flags & MIDX_PROGRESS)
+			progress = start_sparse_progress(_("Verifying OID order in multi-pack-index"),
+							 m->num_objects - 1);
+		for (i = 0; i < m->num_objects - 1; i++) {
+			struct object_id oid1, oid2;
 
-		nth_midxed_object_oid(&oid1, m, i);
-		nth_midxed_object_oid(&oid2, m, i + 1);
+			nth_midxed_object_oid(&oid1, m, i);
+			nth_midxed_object_oid(&oid2, m, i + 1);
 
-		if (oidcmp(&oid1, &oid2) >= 0)
-			midx_report(_("oid lookup out of order: oid[%d] = %s >= %s = oid[%d]"),
-				    i, oid_to_hex(&oid1), oid_to_hex(&oid2), i + 1);
+			if (oidcmp(&oid1, &oid2) >= 0)
+				midx_report(_("oid lookup out of order: oid[%d] = %s >= %s = oid[%d]"),
+					    i, oid_to_hex(&oid1), oid_to_hex(&oid2), i + 1);
 
-		midx_display_sparse_progress(progress, i + 1);
+			midx_display_sparse_progress(progress, i + 1);
+		}
+		stop_progress(&progress);
 	}
-	stop_progress(&progress);
 
 	/*
 	 * Create an array mapping each object to its packfile id.  Sort it
-- 
Patched on top of v2.26.0-rc1-6-ga56d361f66 (git version 2.25.1)