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

[Mark Thompson <sw@jkqxz.net>]

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 problem is mostly harmless because the nondeterministic replacement of structs which we were expecting to be equivalent doesn't actually change anything, so why don't we just remove the comparison and always replace?

Thanks,

- Mark
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