On Mon, 27 Apr 2015 04:50:27 +0200
Michael Niedermayer <michae...@gmx.at> wrote:
> On Tue, Apr 21, 2015 at 02:23:19PM +0200, wm4 wrote:
> > On Tue, 21 Apr 2015 13:22:00 +0200
> > Michael Niedermayer <michae...@gmx.at> wrote:
> >
> > > This indicates that its safe to use av_free/av_malloc on the IO context
> > >
> > > Signed-off-by: Michael Niedermayer <michae...@gmx.at>
> > > ---
> > > libavformat/avio.h | 7 +++++++
> > > libavformat/aviobuf.c | 1 +
> > > libavformat/segment.c | 1 +
> > > libavformat/wtvdec.c | 3 ++-
> > > 4 files changed, 11 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/libavformat/avio.h b/libavformat/avio.h
> > > index 51913e3..73d1645 100644
> > > --- a/libavformat/avio.h
> > > +++ b/libavformat/avio.h
> > > @@ -196,6 +196,13 @@ typedef struct AVIOContext {
> > > * This field is internal to libavformat and access from outside is
> > > not allowed.
> > > */
> > > int orig_buffer_size;
> > > +
> > > + /**
> > > + * The io buffer can be resized or freed with av_free / av_malloc.
> > > + * The user application does not keep a private copy of the buffer
> > > pointer
> > > + * which would become stale on such reallocation.
> > > + */
> > > + int resizeable;
> > > } AVIOContext;
> > >
> > > /* unbuffered I/O */
> > > diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
> > > index 7de59f1..b32ff9f 100644
> > > --- a/libavformat/aviobuf.c
> > > +++ b/libavformat/aviobuf.c
> > > @@ -793,6 +793,7 @@ int ffio_fdopen(AVIOContext **s, URLContext *h)
> > > (*s)->read_seek = (int64_t (*)(void *, int, int64_t,
> > > int))h->prot->url_read_seek;
> > > }
> > > (*s)->av_class = &ff_avio_class;
> > > + (*s)->resizeable = 1;
> > > return 0;
> > > }
> > >
> > > diff --git a/libavformat/segment.c b/libavformat/segment.c
> > > index 1162ea2..6504b46 100644
> > > --- a/libavformat/segment.c
> > > +++ b/libavformat/segment.c
> > > @@ -511,6 +511,7 @@ static int open_null_ctx(AVIOContext **ctx)
> > > av_free(buf);
> > > return AVERROR(ENOMEM);
> > > }
> > > + (*ctx)->resizeable = 1;
> > > return 0;
> > > }
> > >
> > > diff --git a/libavformat/wtvdec.c b/libavformat/wtvdec.c
> > > index e226690..7b5477b 100644
> > > --- a/libavformat/wtvdec.c
> > > +++ b/libavformat/wtvdec.c
> > > @@ -243,7 +243,8 @@ static AVIOContext * wtvfile_open_sector(int
> > > first_sector, uint64_t length, int
> > > av_freep(&buffer);
> > > av_freep(&wf->sectors);
> > > av_freep(&wf);
> > > - }
> > > + } else
> > > + pb->resizeable = 1;
> > > return pb;
> > > }
> > >
> >
> > Looking at it again, ffio_ensure_seekback and ffio_set_buf_size are
> > functions which resize the buffer (but by allocating a new buffer to
> > avoid av_realloc incompatibility, as Nicolas pointed out).
> >
>
> > So what's the point of this patch?
>
> it intends to prevent memory bugs, like double frees, freeing
> randon addresses on the stack and things like that
>
> [...]
Downstream should be fixed (instead of adding such a hack to ffmpeg),
and custom IO should be made possible without forcing the user to do
the tricky buffer allocation. I think this would be ideal. Didn't check
yet why the user needs to allocate a buffer, or how it could be avoided.
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