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seek: Fix crashes in ff_seek_frame_binary if built with latest Clang 14

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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@martin.st>
(cherry picked from commit ab792634197e364ca1bb194f9abe36836e42f12d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
This commit is contained in:
Martin Storsjö 2021-10-18 12:31:38 +03:00 committed by Michael Niedermayer
parent b8d9c50129
commit 3d80573367
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1 changed files with 1 additions and 1 deletions
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libavformat/utils.c
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@ -1870,7 +1870,7 @@ int ff_seek_frame_binary(AVFormatContext *s, int stream_index,
int64_t target_ts, int flags)
{
AVInputFormat *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;