[FFmpeg-devel] [PATCH] seek: Fix crashes in ff_seek_frame_binary if built with latest Clang 14

[Andreas Rheinhardt]

Martin Storsjö:
> Passing an uninitialized variable as argument to a function is
> undefined behaviour (UB). The compiler can assume that UB does not
> happen.
> 
> Hence, the compiler can assume that the variables are never
> uninitialized when passed as argument, which means that the codepaths
> that initializes them must be taken.
> 
> In ff_seek_frame_binary, this means that the compiler can assume
> that the codepaths that initialize pos_min and pos_max are taken,
> which means that the conditions "if (sti->index_entries)" and
> "if (index >= 0)" can be optimized out.
> 
> Current Clang git versions (upcoming Clang 14) enabled an optimization
> that does this, which broke the current version of this function
> (which intentionally left the variables uninitialized, but silencing
> warnings about being uninitialized). See [1] for discussion on
> the matter.
> 
> [1] https://reviews.llvm.org/D105169#3069555
> 
> Signed-off-by: Martin Storsjö <martin at martin.st>
> ---
>  libavformat/seek.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavformat/seek.c b/libavformat/seek.c
> index 40169736df..405ca316b3 100644
> --- a/libavformat/seek.c
> +++ b/libavformat/seek.c
> @@ -283,7 +283,7 @@ int ff_seek_frame_binary(AVFormatContext *s, int stream_index,
>                           int64_t target_ts, int flags)
>  {
>      const AVInputFormat *const avif = s->iformat;
> -    int64_t av_uninit(pos_min), av_uninit(pos_max), pos, pos_limit;
> +    int64_t pos_min = 0, pos_max = 0, pos, pos_limit;
>      int64_t ts_min, ts_max, ts;
>      int index;
>      int64_t ret;
> 

I already wanted to write that the compiler is wrong, but it seems it is
not, as C11 differs from C99 in this respect (C11 6.3.2.1 2):

"If the lvalue designates an
 object of automatic storage duration that could have been declared with
the register storage class (never had its address taken), and that
object is uninitialized (not declared
 with an initializer and no assignment to it has been performed prior to
use), the behavior
 is undefined."

For GCC and Clang av_uninit(x) is defined as x=x. And that is a problem:
In case this macro is used to declare an automatic variable that is
could be declared with the register storage class the
pseudo-initialization is UB according to the above. So I think we will
have to modify the macro to make it safe. Or just stop using it.
(How could such a hack ever end up in a public header?)

- Andreas