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Re: [PATCH] multi-pack-index: make code -Wunused-parameter clean

On 11/3/2018 10:27 PM, Jeff King wrote:
On Sat, Nov 03, 2018 at 05:49:57PM -0700, Carlo Marcelo Arenas Belón wrote:

introduced in 662148c435 ("midx: write object offsets", 2018-07-12)
but included on all previous versions as well.

midx.c:713:54: warning: unused parameter 'nr_objects' [-Wunused-parameter]

likely an oversight as the information needed to iterate over is
embedded in nr_large_offset
I've been preparing a series to make the whole code base compile with
-Wunused-parameter, and I handled this case a bit differently.

-- >8 --
Subject: [PATCH] midx: double-check large object write loop

The write_midx_large_offsets() function takes an array of object
entries, the number of entries in the array (nr_objects), and the number
of entries with large offsets (nr_large_offset). But we never actually
use nr_objects; instead we keep walking down the array and counting down
nr_large_offset until we've seen all of the large entries.

This is correct, but we can be a bit more defensive. If there were ever
a mismatch between nr_large_offset and the actual set of large-offset
objects, we'd walk off the end of the array.

Since we know the size of the array, we can use nr_objects to make sure
we don't walk too far.

Signed-off-by: Jeff King <peff@xxxxxxxx>

Thanks, both, for catching this. I prefer the approach that adds defenses.

Reviewed-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>

  midx.c | 12 +++++++++---
  1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/midx.c b/midx.c
index 4fac0cd08a..ecd583666a 100644
--- a/midx.c
+++ b/midx.c
@@ -712,12 +712,18 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
  static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_offset,
  				       struct pack_midx_entry *objects, uint32_t nr_objects)
-	struct pack_midx_entry *list = objects;
+	struct pack_midx_entry *list = objects, *end = objects + nr_objects;
  	size_t written = 0;
while (nr_large_offset) {
-		struct pack_midx_entry *obj = list++;
-		uint64_t offset = obj->offset;
+		struct pack_midx_entry *obj;
+		uint64_t offset;
+		if (list >= end)
+			BUG("too many large-offset objects");
+		obj = list++;
+		offset = obj->offset;
if (!(offset >> 31))