[FFmpeg-cvslog] fftools/ffmpeg_filter: Avoid DynBuf API to improve error checks

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Wed Dec  1 17:13:32 2021 +0100| [efc323062c20aaead8fb5805b7f69b4f071cb319] | committer: Andreas Rheinhardt

fftools/ffmpeg_filter: Avoid DynBuf API to improve error checks

choose_pix_fmts() used the dynamic buffer API to write strings;
as is common among uses of this API, only opening the dynamic buffer
was checked, but not the end result, leading to crashes in case
of allocation failure.
Furthermore, some static strings were duplicated; the allocations
performed here were not properly checked: Allocation failure would
be treated as "could not determine pixel format".
The first issue is fixed by switching to the AVBPrint API which allows
to easily perform checks at the end. Furthermore, the internal buffer
avoids almost all allocations in case the AVBPrint is used.
The AVBPrint also allows to solve the second issue in an elegant way,
because it allows to return the static strings directly.

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

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=efc323062c20aaead8fb5805b7f69b4f071cb319
---

 fftools/ffmpeg_filter.c | 31 +++++++++++++++----------------
 1 file changed, 15 insertions(+), 16 deletions(-)

diff --git a/fftools/ffmpeg_filter.c b/fftools/ffmpeg_filter.c
index 47bbb67ce0..dab0f28819 100644
--- a/fftools/ffmpeg_filter.c
+++ b/fftools/ffmpeg_filter.c
@@ -83,7 +83,10 @@ static enum AVPixelFormat choose_pixel_fmt(AVStream *st, AVCodecContext *enc_ctx
     return target;
 }
 
-static char *choose_pix_fmts(OutputFilter *ofilter)
+/* May return NULL (no pixel format found), a static string or a string
+ * backed by the bprint. Nothing has been written to the AVBPrint in case
+ * NULL is returned. The AVBPrint provided should be clean. */
+static const char *choose_pix_fmts(OutputFilter *ofilter, AVBPrint *bprint)
 {
     OutputStream *ost = ofilter->ost;
     const AVDictionaryEntry *strict_dict = av_dict_get(ost->encoder_opts, "strict", NULL, 0);
@@ -96,18 +99,12 @@ static char *choose_pix_fmts(OutputFilter *ofilter)
                                             AVFILTER_AUTO_CONVERT_NONE);
         if (ost->enc_ctx->pix_fmt == AV_PIX_FMT_NONE)
             return NULL;
-        return av_strdup(av_get_pix_fmt_name(ost->enc_ctx->pix_fmt));
+        return av_get_pix_fmt_name(ost->enc_ctx->pix_fmt);
     }
     if (ost->enc_ctx->pix_fmt != AV_PIX_FMT_NONE) {
-        return av_strdup(av_get_pix_fmt_name(choose_pixel_fmt(ost->st, ost->enc_ctx, ost->enc, ost->enc_ctx->pix_fmt)));
+        return av_get_pix_fmt_name(choose_pixel_fmt(ost->st, ost->enc_ctx, ost->enc, ost->enc_ctx->pix_fmt));
     } else if (ost->enc && ost->enc->pix_fmts) {
         const enum AVPixelFormat *p;
-        AVIOContext *s = NULL;
-        uint8_t *ret;
-        int len;
-
-        if (avio_open_dyn_buf(&s) < 0)
-            exit_program(1);
 
         p = ost->enc->pix_fmts;
         if (ost->enc_ctx->strict_std_compliance > FF_COMPLIANCE_UNOFFICIAL) {
@@ -116,11 +113,11 @@ static char *choose_pix_fmts(OutputFilter *ofilter)
 
         for (; *p != AV_PIX_FMT_NONE; p++) {
             const char *name = av_get_pix_fmt_name(*p);
-            avio_printf(s, "%s|", name);
+            av_bprintf(bprint, "%s%c", name, p[1] == AV_PIX_FMT_NONE ? '\0' : '|');
         }
-        len = avio_close_dyn_buf(s, &ret);
-        ret[len - 1] = 0;
-        return ret;
+        if (!av_bprint_is_complete(bprint))
+            exit_program(1);
+        return bprint->str;
     } else
         return NULL;
 }
@@ -416,12 +413,13 @@ static int insert_filter(AVFilterContext **last_filter, int *pad_idx,
 
 static int configure_output_video_filter(FilterGraph *fg, OutputFilter *ofilter, AVFilterInOut *out)
 {
-    char *pix_fmts;
     OutputStream *ost = ofilter->ost;
     OutputFile    *of = output_files[ost->file_index];
     AVFilterContext *last_filter = out->filter_ctx;
+    AVBPrint bprint;
     int pad_idx = out->pad_idx;
     int ret;
+    const char *pix_fmts;
     char name[255];
 
     snprintf(name, sizeof(name), "out_%d_%d", ost->file_index, ost->index);
@@ -457,13 +455,14 @@ static int configure_output_video_filter(FilterGraph *fg, OutputFilter *ofilter,
         pad_idx = 0;
     }
 
-    if ((pix_fmts = choose_pix_fmts(ofilter))) {
+    av_bprint_init(&bprint, 0, AV_BPRINT_SIZE_UNLIMITED);
+    if ((pix_fmts = choose_pix_fmts(ofilter, &bprint))) {
         AVFilterContext *filter;
 
         ret = avfilter_graph_create_filter(&filter,
                                            avfilter_get_by_name("format"),
                                            "format", pix_fmts, NULL, fg->graph);
-        av_freep(&pix_fmts);
+        av_bprint_finalize(&bprint, NULL);
         if (ret < 0)
             return ret;
         if ((ret = avfilter_link(last_filter, pad_idx, filter, 0)) < 0)