[FFmpeg-devel] [PR] avcodec/mlp: Don't use internals of CRC API (PR #21507)

[FFmpeg-devel] [PR] avcodec/mlp: Don't use internals of CRC API (PR #21507)

[mkver via ffmpeg-devel]

PR #21507 opened by mkver
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21507
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21507.patch

ff_mlp_restart_checksum() used the (undocumented) layout
of the CRC tables and therefore broke on x86 when the
clmul implementation added in dc03cffe9c9577127ef82b6f56118115f900e5f2
is used. This commit fixes this.


>From f330d178adf5508607f2c1f3d0cc0201b6fd4d18 Mon Sep 17 00:00:00 2001
From: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Date: Mon, 19 Jan 2026 00:26:34 +0100
Subject: [PATCH] avcodec/mlp: Don't use internals of CRC API

ff_mlp_restart_checksum() used the (undocumented) layout
of the CRC tables and therefore broke on x86 when the
clmul implementation added in dc03cffe9c9577127ef82b6f56118115f900e5f2
is used. This commit fixes this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
 libavcodec/mlp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/libavcodec/mlp.c b/libavcodec/mlp.c
index 33045c08c7..8ea6e6d72e 100644
--- a/libavcodec/mlp.c
+++ b/libavcodec/mlp.c
@@ -107,8 +107,11 @@ uint8_t ff_mlp_restart_checksum(const uint8_t *buf, unsigned int bit_size)
     int i;
     int num_bytes = (bit_size + 2) / 8;
 
-    int crc = crc_1D[buf[0] & 0x3f];
-    crc = av_crc(crc_1D, crc, buf + 1, num_bytes - 2);
+    // The two most significant bits of buf[0] are not supposed
+    // to be contained in the checksum; using buf[0] & 0xC0 as start value
+    // achieves this.
+    int crc = av_crc(crc_1D, buf[0] & 0xC0, buf, num_bytes - 1);
+
     crc ^= buf[num_bytes - 1];
 
     for (i = 0; i < ((bit_size + 2) & 7); i++) {
-- 
2.52.0

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