Re: [FFmpeg-devel] [PATCH] avcodec/hevc_ps: fix the problem of memcmp losing effectiveness

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Mark Thompson:
> On 29/03/2024 14:00, Andreas Rheinhardt wrote:
>> James Almer:
>>> On 3/29/2024 10:10 AM, Mark Thompson wrote:
>>>> On 28/03/2024 13:15, tong1.wu-at-intel.com@ffmpeg.org wrote:
>>>>> From: Tong Wu <tong1.wu@intel.com>
>>>>>
>>>>> HEVCHdrParams* receives a pointer which points to a dynamically
>>>>> allocated memory block. It causes the memcmp always returning 1.
>>>>> Add a function to do the comparision. A condition is also added to
>>>>> avoid malloc(0).
>>>>>
>>>>> Signed-off-by: Tong Wu <tong1.wu@intel.com>
>>>>> ---
>>>>>    libavcodec/hevc_ps.c | 20 ++++++++++++++++----
>>>>>    libavcodec/hevc_ps.h |  4 +++-
>>>>>    2 files changed, 19 insertions(+), 5 deletions(-)
>>>>
>>>> It doesn't seem like this method works at all, even before the recent
>>>> change with the pointer.
>>>>
>>>> Structs can contain arbitrary padding, and any write to the struct
>>>> makes the padding unspecified.  memcmp() is therefore never valid as a
>>>> method of comparing after writing some fields, as done here.  (It
>>>> could only be valid if the structs compared were made by memcpy() with
>>>> no fields written directly.)
>>>
>>> The struct is zero allocated, so shouldn't the padding be exactly the
>>> same for two equal VPSs after parsing?
>>>
>>
>> In practice it is (and the current code already relied on this); yet as
>> has already been said padding bytes take unspecified values at any store
>> (to any member). In practice, if the compiler uses instructions that
>> clobber the padding, the padding in both structs is clobbered in the
>> same way.
> 
> This seems like a strong assumption beyond that of the C specification
> which needs to be documented.
> 
> Are you expecting that there is no case where ABI-undefined top bits of
> registers can leak into the padding fields, or that all functions called
> here will necessarily set those top bits to the same values, or
> something else?
> 

Yes, I am expecting that. That is also what the code already relied on
before the addition of the allocated field and there have been no
reports that this caused issues.
This does not change that I consider it crazy to remove the parameter
sets referencing a parameter set that is removed.

- Andreas

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