| Message ID | 20230131111349.1338846-1-tim@ngus.net |
|---|---|
| State | New |
| Headers | show |
| Series | None | expand |
On 31/01/2023 11:13, Tim Angus wrote: > In the write_header function of the ASS encoder, extradata is > searched for a substring using the strstr function. strstr expects a > null terminated C string as its first parameter, but extradata is > (notionally) not one of these, meaning that the calls to strstr only > happen to work because (presumably) they encounter a 0 byte somewhere > downstream of extradata. I say notionally because often extradata will > in fact be null terminated (which causes other problems, see parent > commit). > > Signed-off-by: Tim Angus <tim@ngus.net> > --- > libavformat/assenc.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/libavformat/assenc.c b/libavformat/assenc.c > index 4c9ea6f982..f6f5293bb7 100644 > --- a/libavformat/assenc.c > +++ b/libavformat/assenc.c > @@ -83,12 +83,12 @@ static int write_header(AVFormatContext *s) > if (header_string[strlen(header_string) - 1] != '\n') > avio_printf(s->pb, "\r\n"); > > - av_free(header_string); > - > - ass->ssa_mode = !strstr(par->extradata, "\n[V4+ Styles]"); > - if (!strstr(par->extradata, "\n[Events]")) > + ass->ssa_mode = !strstr(header_string, "\n[V4+ Styles]"); > + if (!strstr(header_string, "\n[Events]")) > avio_printf(s->pb, "[Events]\r\nFormat: %s, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text\r\n", > ass->ssa_mode ? "Marked" : "Layer"); > + > + av_free(header_string); > } > > return 0; Apparently extradata is padded with zeroes, so this is probably unnecessary, strictly speaking. Having said that, reasoning about why the code works is less obscure with the patch applied.
diff --git a/libavformat/assenc.c b/libavformat/assenc.c index 4c9ea6f982..f6f5293bb7 100644 --- a/libavformat/assenc.c +++ b/libavformat/assenc.c @@ -83,12 +83,12 @@ static int write_header(AVFormatContext *s) if (header_string[strlen(header_string) - 1] != '\n') avio_printf(s->pb, "\r\n"); - av_free(header_string); - - ass->ssa_mode = !strstr(par->extradata, "\n[V4+ Styles]"); - if (!strstr(par->extradata, "\n[Events]")) + ass->ssa_mode = !strstr(header_string, "\n[V4+ Styles]"); + if (!strstr(header_string, "\n[Events]")) avio_printf(s->pb, "[Events]\r\nFormat: %s, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text\r\n", ass->ssa_mode ? "Marked" : "Layer"); + + av_free(header_string); } return 0;
In the write_header function of the ASS encoder, extradata is searched for a substring using the strstr function. strstr expects a null terminated C string as its first parameter, but extradata is (notionally) not one of these, meaning that the calls to strstr only happen to work because (presumably) they encounter a 0 byte somewhere downstream of extradata. I say notionally because often extradata will in fact be null terminated (which causes other problems, see parent commit). Signed-off-by: Tim Angus <tim@ngus.net> --- libavformat/assenc.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)