[FFmpeg-cvslog] avformat/allformats: Fix data race when accessing devices lists

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Fri Oct  1 18:17:42 2021 +0200| [5a261cfa32d2833d20d87414bef267f3765abc4d] | committer: Andreas Rheinhardt

avformat/allformats: Fix data race when accessing devices lists

Up until now setting the input and output devices lists is guarded
by a mutex. This prevents data races emanating from multiple concurrent
calls to avpriv_register_devices() (triggered by multiple concurrent
calls to avdevice_register_all()). Yet reading the lists pointers was
done without any lock and with nonatomic variables. This means that
there are data races in case of concurrent calls to
av_(de)muxer_iterate() and avdevice_register_all() (but only if the
iteration in av_(de)muxer_iterate exhausts the non-device (de)muxers).

This commit fixes this by putting said pointers into atomic objects.
Due to the unavailability of _Atomic the object is an atomic_uintptr,
leading to ugly casts. Switching to atomics also allowed to remove
the mutex currently used in avpriv_register_devices().

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=5a261cfa32d2833d20d87414bef267f3765abc4d
---

 libavformat/allformats.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/libavformat/allformats.c b/libavformat/allformats.c
index 99d8c91e00..889cfcc2c9 100644
--- a/libavformat/allformats.c
+++ b/libavformat/allformats.c
@@ -19,7 +19,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include "libavutil/thread.h"
+#include <stdatomic.h>
 #include "libavformat/internal.h"
 #include "avformat.h"
 
@@ -535,18 +535,20 @@ extern const AVInputFormat  ff_vapoursynth_demuxer;
 #include "libavformat/muxer_list.c"
 #include "libavformat/demuxer_list.c"
 
-static const AVInputFormat * const *indev_list = NULL;
-static const AVOutputFormat * const *outdev_list = NULL;
+static atomic_uintptr_t indev_list_intptr  = ATOMIC_VAR_INIT(0);
+static atomic_uintptr_t outdev_list_intptr = ATOMIC_VAR_INIT(0);
 
 const AVOutputFormat *av_muxer_iterate(void **opaque)
 {
     static const uintptr_t size = sizeof(muxer_list)/sizeof(muxer_list[0]) - 1;
     uintptr_t i = (uintptr_t)*opaque;
     const AVOutputFormat *f = NULL;
+    uintptr_t tmp;
 
     if (i < size) {
         f = muxer_list[i];
-    } else if (outdev_list) {
+    } else if (tmp = atomic_load_explicit(&outdev_list_intptr, memory_order_relaxed)) {
+        const AVOutputFormat *const *outdev_list = (const AVOutputFormat *const *)tmp;
         f = outdev_list[i - size];
     }
 
@@ -560,10 +562,12 @@ const AVInputFormat *av_demuxer_iterate(void **opaque)
     static const uintptr_t size = sizeof(demuxer_list)/sizeof(demuxer_list[0]) - 1;
     uintptr_t i = (uintptr_t)*opaque;
     const AVInputFormat *f = NULL;
+    uintptr_t tmp;
 
     if (i < size) {
         f = demuxer_list[i];
-    } else if (indev_list) {
+    } else if (tmp = atomic_load_explicit(&indev_list_intptr, memory_order_relaxed)) {
+        const AVInputFormat *const *indev_list = (const AVInputFormat *const *)tmp;
         f = indev_list[i - size];
     }
 
@@ -572,12 +576,8 @@ const AVInputFormat *av_demuxer_iterate(void **opaque)
     return f;
 }
 
-static AVMutex avpriv_register_devices_mutex = AV_MUTEX_INITIALIZER;
-
 void avpriv_register_devices(const AVOutputFormat * const o[], const AVInputFormat * const i[])
 {
-    ff_mutex_lock(&avpriv_register_devices_mutex);
-    outdev_list = o;
-    indev_list = i;
-    ff_mutex_unlock(&avpriv_register_devices_mutex);
+    atomic_store_explicit(&outdev_list_intptr, (uintptr_t)o, memory_order_relaxed);
+    atomic_store_explicit(&indev_list_intptr,  (uintptr_t)i, memory_order_relaxed);
 }