Re: [FFmpeg-devel] [PATCH] avcodec/vp9mvs: fix misaligned access when clearing VP9mv

[Michael Niedermayer <michael@niedermayer.cc>]

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On Sun, Jun 02, 2024 at 07:05:37PM -0400, Ronald S. Bultje wrote:
> Hi,
> 
> On Sun, Jun 2, 2024 at 6:43 PM Kacper Michajlow <kasper93@gmail.com> wrote:
> 
> > On Sun, 2 Jun 2024 at 23:17, Ronald S. Bultje <rsbultje@gmail.com> wrote:
> > >
> > > Hi,
> > >
> > > On Sun, Jun 2, 2024 at 9:12 AM James Almer <jamrial@gmail.com> wrote:
> > >
> > > > On 6/2/2024 10:06 AM, James Almer wrote:
> > > > > On 6/2/2024 9:14 AM, Kacper Michajłow wrote:
> > > > >> Fixes runtime error: member access within misaligned address
> > > > >> <addr> for type 'av_alias64', which requires 8 byte alignment.
> > > > >>
> > > > >> VP9mv is aligned to 4 bytes, so instead doing 8 bytes clear, let's
> > do
> > > > >> 2 times 4 bytes.
> > > > >>
> > > > >> Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
> > > > >> ---
> > > > >>   libavcodec/vp9mvs.c | 3 ++-
> > > > >>   1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >>
> > > > >> diff --git a/libavcodec/vp9mvs.c b/libavcodec/vp9mvs.c
> > > > >> index b706d1660f..790cf629a6 100644
> > > > >> --- a/libavcodec/vp9mvs.c
> > > > >> +++ b/libavcodec/vp9mvs.c
> > > > >> @@ -294,7 +294,8 @@ void ff_vp9_fill_mv(VP9TileData *td, VP9mv *mv,
> > > > >> int mode, int sb)
> > > > >>       VP9Block *b = td->b;
> > > > >>       if (mode == ZEROMV) {
> > > > >> -        AV_ZERO64(mv);
> > > > >> +        AV_ZERO32(&mv[0]);
> > > > >> +        AV_ZERO32(&mv[1]);
> > > > >>       } else {
> > > > >>           int hp;
> > > > >
> > > > > IMO just move mv in VP9Block to the top of the struct. That will make
> > > > > sure it's aligned to at the very least 16 byte (Since it's
> > av_malloc'd).
> > > >
> > > > Actually nevermind, VP9mv has two int16_t and given what's passed to
> > > > ff_vp9_fill_mv() it's not enough.
> > > >
> > >
> > > Do compilers on relevant platforms convert this to a single 64bit
> > > (unaligned) zero-move? Otherwise, we may want an unaligned AV_ZERO64() so
> > > as to not slow down platforms supporting unaligned writes.
> >
> > Yes, exactly, compilers do that. I've checked before sending this
> > patch if it doesn't do something overly silly.
> >
> > You can play around here to see, I've extracted relevant part
> > https://godbolt.org/z/K4d7Ejb1P
> >
> 
> Thanks for checking, patch is fine with me.

will apply

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you drop bombs on a foreign country and kill a hundred thousand
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