[FFmpeg-devel] [PATCH v2 2/2] avformat/mov: Don't use entry[-1] in pointer arithmetic

[Andreas Rheinhardt]

It is undefined behaviour.
Fixes many failed tests with UBSan and GCC 13 like
"src/libavformat/mov.c:4229:44: runtime error: store to address
0x5572abe20f80 with insufficient space for an object of type 'struct
MOVIndexRange'"
(The line number does not refer to the line where &entry[-1]
is assigned.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
---
Now switching to next_index_range in order to avoid
having to duplicate the current_index_range == NULL check.

 libavformat/mov.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/libavformat/mov.c b/libavformat/mov.c
index 71e8f7ae8f..d7da19998b 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -4022,7 +4022,7 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
     int num_discarded_begin = 0;
     int first_non_zero_audio_edit = -1;
     int packet_skip_samples = 0;
-    MOVIndexRange *current_index_range;
+    MOVIndexRange *next_index_range;
     int found_keyframe_after_edit = 0;
     int found_non_empty_edit = 0;
 
@@ -4038,7 +4038,7 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
         return;
     }
     msc->current_index_range = msc->index_ranges;
-    current_index_range = msc->index_ranges - 1;
+    next_index_range = msc->index_ranges;
 
     // Clean AVStream from traces of old index
     sti->index_entries = NULL;
@@ -4225,11 +4225,11 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
             }
 
             // Update the index ranges array
-            if (current_index_range < msc->index_ranges || index != current_index_range->end) {
-                current_index_range++;
-                current_index_range->start = index;
+            if (next_index_range == msc->index_ranges || index != next_index_range[-1].end) {
+                next_index_range->start = index;
+                next_index_range++;
             }
-            current_index_range->end = index + 1;
+            next_index_range[-1].end = index + 1;
 
             // Only start incrementing DTS in frame_duration amounts, when we encounter a frame in edit list.
             if (edit_list_start_encountered > 0) {
@@ -4289,9 +4289,8 @@ static void mov_fix_index(MOVContext *mov, AVStream *st)
     av_freep(&frame_duration_buffer);
 
     // Null terminate the index ranges array
-    current_index_range++;
-    current_index_range->start = 0;
-    current_index_range->end = 0;
+    next_index_range->start = 0;
+    next_index_range->end   = 0;
     msc->current_index = msc->index_ranges[0].start;
 }
 
-- 
2.40.1