[FFmpeg-devel] [PATCH 2/2] avcodec/avdct: Clear IDCTDSPContext context

[James Almer]

On 1/28/2020 10:30 AM, Michael Niedermayer wrote:
> On Mon, Jan 27, 2020 at 11:49:49PM -0300, James Almer wrote:
>> On 1/27/2020 9:25 PM, Michael Niedermayer wrote:
>>> On Mon, Jan 27, 2020 at 06:09:28PM -0300, James Almer wrote:
>>>> On 1/27/2020 5:54 PM, Michael Niedermayer wrote:
>>>>> Fixes use of uninitialized variable and segfault
>>>>>
>>>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>>>> ---
>>>>>  libavcodec/avdct.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/libavcodec/avdct.c b/libavcodec/avdct.c
>>>>> index 47e5f7134e..7c761cf39a 100644
>>>>> --- a/libavcodec/avdct.c
>>>>> +++ b/libavcodec/avdct.c
>>>>> @@ -100,7 +100,7 @@ int avcodec_dct_init(AVDCT *dsp)
>>>>>  
>>>>>  #if CONFIG_IDCTDSP
>>>>>      {
>>>>> -        IDCTDSPContext idsp;
>>>>> +        IDCTDSPContext idsp = {0};
>>>>
>>>> Should probably be a memset() in ff_idctdsp_init(). This is not the only
>>>> IDCTDSPContext user.
>>>
>>> this would not work as IDCTDSPContext.mpeg4_studio_profile must be 
>>> initialized but it is also an input to ff_idctdsp_init()
>>>
>>> an alternative to the = {0} on the caller side would be to
>>> simply add the mpeg4_studio_profile as an argument to ff_idctdsp_init()
>>> and remove it from the context, its all just internal API so we can
>>> easily redesign this. It should also be documented better ...
>>>
>>> What do you suggest ?
>>
>> memset(c, 0, offsetof(IDCTDSPContext, mpeg4_studio_profile)) in
>> ff_idctdsp_init() should workaround that without the need to add new
>> parameters. Just document that mpeg4_studio_profile must be at the end
>> of the struct, and be the first field of all those that must be
>> initialized before a call to ff_idctdsp_init(), in case new ones are
>> added in the future for whatever reason.
> 
> The memset() suggested in ff_idctdsp_init() would make no difference
> as the function already sets all fields which are not an input.
> This would reduce the suggestion to setting mpeg4_studio_profile
> in the ?single? caller which does not memset(0) the context prior.
> 
> That leaves 3 suggestions
> A. "-        IDCTDSPContext idsp;"
> A. "+        IDCTDSPContext idsp = {0};"     (for one caller)
> 
> B. "+        idsp.mpeg4_studio_profile = 0;" (for one caller)
> 
> C. "-        ff_idctdsp_init();"   
> C. "+        ff_idctdsp_init(0);"            (for all callers)
> 
> In all cases ff_idctdsp_init() also probably should be documented.
> 
> What do you prefer ?

In that case just go with A, your original patch.

> 
> thx
> 
> [...]
> 
> 
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