Re: [FFmpeg-devel] [PATCH v3] avformat/avisynth: fix segfault when also compiled with libvmaf

[Kacper Michajlow <kasper93@gmail.com>]

On Mon, 28 Jul 2025 at 02:04, Daniel Bermond <danielbermond@gmail.com> wrote:
>
> When FFmpeg is compiled with support for both avisynth and libvmaf,
> a segmentation fault occurs when using avisynth (.avs) input.
>
> This happens because both avisynthplus and vmaf have the exactly
> same C++ symbol 'Cache::~Cache()'[1][2], which is a C++ destructor.
> When using avisynth input, this destructor is also called in the
> vmaf side, although no vmaf object was created, thus causing a
> segmentation fault.
>
> Without changing this conflicting symbol in avisynthplus and/or
> vmaf upstream code, a solution is to switch the 'RTLD_LOCAL'[3]

I'm not opposed to a workaround to the issues as an interim solution,
but has this issue been reported to upstream? Classes with such
generic names should be put in a namespace in the respective projects.
Additionally it would be good for those projects to be built with
-fvisibility=hidden to reduce the amount of exposed symbols and only
expose API entry points.

- Kacper

> flag for 'RTLD_DEEPBIND'[4] when calling 'dlopen()' on the
> avisynthplus library. This will make the avisynthplus symbols
> loaded by 'dlopen()' to be ahead of the symbols loaded in the
> global scope, and consequently avoid the segmentation fault by
> preventing the conflicting vmaf C++ destructor to be called.
>
> More details can be seen in the discussion of the Arch Linux bug
> report[5] about this issue.
>
> [1] https://github.com/AviSynth/AviSynthPlus/blob/v3.7.5/avs_core/core/cache.h#L62
> [2] https://github.com/Netflix/vmaf/blob/v3.0.0/libvmaf/src/svm.cpp#L75
> [3] https://man.archlinux.org/man/dlopen.3#RTLD_LOCAL
> [4] https://man.archlinux.org/man/dlopen.3#RTLD_DEEPBIND
> [5] https://gitlab.archlinux.org/archlinux/packaging/packages/ffmpeg/-/issues/19
>
> Signed-off-by: Daniel Bermond <danielbermond@gmail.com>
> ---
>  libavformat/avisynth.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libavformat/avisynth.c b/libavformat/avisynth.c
> index cb2be10925..bef61b4a60 100644
> --- a/libavformat/avisynth.c
> +++ b/libavformat/avisynth.c
> @@ -120,7 +120,7 @@ typedef struct AviSynthContext {
>
>  static av_cold int avisynth_load_library(AviSynthContext *avs)
>  {
> -    avs->avs_library.library = dlopen(AVISYNTH_LIB, RTLD_NOW | RTLD_LOCAL);
> +    avs->avs_library.library = dlopen(AVISYNTH_LIB, RTLD_NOW | RTLD_DEEPBIND);
>      if (!avs->avs_library.library)
>          return AVERROR_UNKNOWN;
>
> --
> 2.50.1
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