Re: [FFmpeg-devel] [PATCH v3] libavcodec: Set hidden visibility on global symbols accessed from AArch64 assembly

["Martin Storsjö" <martin@martin.st>]

On Thu, 14 Jul 2022, Martin Storsjö wrote:

> The AArch64 assembly accesses those symbols directly, without
> indirection via e.g. the GOT on ELF. In order for this not to
> require text relocations, those symbols need to be resolved fully
> at link time, i.e. those symbols can't be interposable.
>
> Normally, so far, this is achieved when linking shared libraries
> in two ways; we have a version script (libavcodec/libavcodec.v) which
> marks all symbols that don't start with av* as local. Additionally,
> we try to add -Wl,-Bsymbolic to the linker options if supported,
> making sure that such symbol references are resolved fully at link
> time, instead of making them interposable.
>
> When the libavcodec static library is linked into another shared
> library, there's no guarantee that it uses similar options (even though
> that would be favourable), which would end up requiring text relocations
> in the AArch64 assembly.
>
> Explicitly mark the symbols that are accessed from AArch64 assembly
> as hidden, so that they are resolved fully at link time even without
> the version script and -Wl,-Bsymbolic.
>
> Signed-off-by: Martin Storsjö <martin@martin.st>
> ---
> Moved the attribute to libavutil/internal.h, renamed to a different
> namespace (not av_ prefixed), moved the attribute on ff_vp9_subpel_filters
> to the header, as suggested.
>
> Added explicit includes of libavutil/internal.h to fix builds on
> x86.
> ---
> libavcodec/aacsbrdata.h | 3 ++-
> libavcodec/fft.h        | 3 ++-
> libavcodec/vp9dsp.h     | 3 ++-
> libavutil/internal.h    | 6 ++++++
> 4 files changed, 12 insertions(+), 3 deletions(-)

Ping - any further comments on this - is it acceptable/tolerable, as a 
practical fix to make sure these symbols aren't interposable?

// Martin
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