[FFmpeg-devel] [PATCH 1/2] avcodec/h274: don't read from uninitialized array members

[Niklas Haas]

From: Niklas Haas <git at haasn.dev>

This bug flew under the radar because, in practice, these values are
0-initialized for the first invocation. But for subsequent invocations
(with different h/v values), reading from the uninitialized parts of
`out` is undefined behavior.

Avoid this by simply adjusting the iteration range of the next loop. Has
the added benefit of being a minor speedup.
---
 libavcodec/h274.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libavcodec/h274.c b/libavcodec/h274.c
index 5e2cf150ea..a59d09b66e 100644
--- a/libavcodec/h274.c
+++ b/libavcodec/h274.c
@@ -74,12 +74,14 @@ static void init_slice_c(int8_t out[64][64], uint8_t h, uint8_t v,
 
     // 64x64 inverse integer transform
     for (int y = 0; y < 64; y++) {
-        for (int x = 0; x < 64; x++) {
+        for (int x = 0; x <= freq_h; x++) {
             int32_t sum = 0;
-            for (int p = 0; p < 64; p++)
+            for (int p = 0; p <= freq_v; p++)
                 sum += R64T[y][p] * out[x][p];
             tmp[y][x] = (sum + 128) >> 8;
         }
+        for (int x = freq_h+1; x < 64; x++)
+            tmp[y][x] = 0;
     }
 
     for (int y = 0; y < 64; y++) {
-- 
2.33.0