[MPlayer-dev-eng] Segfault caused by the "expand" filter

[Zuxy Meng]

2011/3/17 Reimar Döffinger <Reimar.Doeffinger at gmx.de>:
> On Wed, Mar 16, 2011 at 06:19:04PM +0800, Zuxy Meng wrote:
>> 2011/3/16 Reimar Döffinger <Reimar.Doeffinger at gmx.de>:
>> > On 15 Mar 2011, at 03:57, Zuxy Meng <zuxy.meng at gmail.com> wrote:
>> >> Hope here's still atmosphere for technical discussions:
>> >>
>> >> I met segfaults triggered by unaligned accesses of SSE instructions.
>> >> The root cause is within the get_image() function of the "expand"
>> >> filter, in lines like:
>> >>
>> >>    if(mpi->flags&MP_IMGFLAG_PLANAR){
>> >>        mpi->planes[0]=vf->dmpi->planes[0]+
>> >>        vf->priv->exp_y*vf->dmpi->stride[0]+vf->priv->exp_x;
>> >>
>> >> Here vf->dmpi->planes[0] is 16-byte aligned, but mpi->planes[0] may not be.
>> >>
>> >> I'm not sure what's the correct fix here. Would a simple forced align
>> >> before the assignment work (e.g. ((vf->dmpi->planes[0] + ...)&~15))?
>> >
>> > To solve it properly you have to allocate a new image and memcpy instead of using EXPORT image type (or is this the DR path? Disable DR in that case).
>>
>> Yes I guess it's the DR path.
>>
>> > Alternatively filters requiring the alignment could do the memcpy, but I think that's more effort for little gain.
>>
>> The problem happens when expand isn't the last filter, e.g. vf=expand...,pp=ac
>
> That doesn't make sense, the line you quoted only causes the filters (and decoder)
> _before_ to get unaligned pointers.

I'm puzzled too. But in my case vf=pp=ac,expand=::::1:8/5 works and
vf=expand=::::1:8/5,pp=ac crashes.

> And I actually think that this code is actually correct, however it might be
> necessary to change MP_IMGFLAG_ACCEPT_STRIDE to MP_IMGFLAG_ACCEPT_ALIGNED_STRIDE
> in vd_ffmpeg.c.
> As an optimization vf_expand could be changed to also work with MP_IMGFLAG_ACCEPT_ALIGNED_STRIDE
> if vf->priv->exp_y*vf->dmpi->stride[0]+vf->priv->exp_x is a multiple of 16 for example.


-- 
Zuxy
Beauty is truth,
While truth is beauty.
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