[Ffmpeg-devel] [PATCH] ASSERT_BITS

[Michael Niedermayer]

Hi

On Wed, Nov 01, 2006 at 09:50:50PM +0100, Steve Lhomme wrote:
> Michael Niedermayer wrote:
> >Hi
> >
> >On Tue, Oct 24, 2006 at 06:00:22PM +0200, Steve Lhomme wrote:
> >>Hi,
> >>
> >>This patch allow to put asserts inside the bitstream reader to make sure 
> >>no bits are used outside of what's in memory. (in the past it helped me 
> >>debug a few buffer overflow).
> >
> >[...]
> >>@@ -497,6 +503,9 @@
> >>         (gb)->cache= name##_cache;\
> >>         (gb)->buffer_ptr= name##_buffer_ptr;\
> >> 
> >>+#   define ASSERT_BITS(gb, num)\
> >>+        assert((gb)->bit_count + FF_INPUT_BUFFER_PADDING_SIZE*8 >= 
> >>(num));
> >>+
> >> #ifdef LIBMPEG2_BITSTREAM_READER_HACK
> >> 
> >> #   define UPDATE_CACHE(name, gb)\
> >>@@ -571,6 +580,9 @@
> >>         (gb)->cache1= name##_cache1;\
> >>         (gb)->buffer_ptr= name##_buffer_ptr;\
> >> 
> >>+#   define ASSERT_BITS(gb, num)\
> >>+        assert((gb)->bit_count + FF_INPUT_BUFFER_PADDING_SIZE*8 >= 
> >>(num));
> >>+
> >> #   define UPDATE_CACHE(name, gb)\
> >>     if(name##_bit_count > 0){\
> >>         const uint32_t next= be2me_32( *name##_buffer_ptr );\
> >
> >these 2 dont look correct, i would rather expect them to compare buffer_ptr
> >against buffer_end
> >also why dont you check in and only in UPDATE_CACHE() ? IIRC thats the only
> >spot where memory gets read (=where a segfault can occur) 
> 
> As I don't have any way to test the other 2, I just add it for 
> ALT_BITSTREAM_READER.

well, you just have to change the define which selects the default reader


> 
> I call it before 'some' calls to UPDATE_CACHE, not all. But I don't 
> think it's good to call it directly inside UPDATE_CACHE, otherwise it 
> might degrade performance a lot. Now if someone wants to improve it, 
> feel free :)

people who care about speed wont have it enabled others will likely want
all cases checked

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In the past you could go to a library and read, borrow or copy any book
Today you'd get arrested for mere telling someone where the library is