Re: [FFmpeg-devel] [PATCH v2] libavcodec: Set hidden visibility on global symbols accessed from AArch64 assembly

[Michael Niedermayer <michael@niedermayer.cc>]

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On Thu, Jul 14, 2022 at 12:14:38AM +0300, 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.
> ---
>  libavcodec/aacsbrdata.h | 2 +-
>  libavcodec/fft.h        | 2 +-
>  libavcodec/vp9dsp.h     | 2 +-
>  libavutil/internal.h    | 6 ++++++
>  4 files changed, 9 insertions(+), 3 deletions(-)

This seems to break build on linux x86-64, i guess iam missing something

MAN	doc/ffprobe-all.1
In file included from libavcodec/x86/vp9dsp_init_16bpp.c:26:0:
./libavcodec/vp9dsp.h:123:50: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘ff_vp9_subpel_filters’
 extern const int16_t attribute_visibility_hidden ff_vp9_subpel_filters[3][16][8];
                                                  ^~~~~~~~~~~~~~~~~~~~~
ffbuild/common.mak:81: recipe for target 'libavcodec/x86/vp9dsp_init_16bpp.o' failed
make: *** [libavcodec/x86/vp9dsp_init_16bpp.o] Error 1
make: *** Waiting for unfinished jobs....
In file included from libavcodec/x86/vp9dsp_init_16bpp_template.c:26:0,
                 from libavcodec/x86/vp9dsp_init_12bpp.c:25:
./libavcodec/vp9dsp.h:123:50: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘ff_vp9_subpel_filters’
 extern const int16_t attribute_visibility_hidden ff_vp9_subpel_filters[3][16][8];
                                                  ^~~~~~~~~~~~~~~~~~~~~
In file included from libavcodec/x86/vp9dsp_init_16bpp_template.c:26:0,
                 from libavcodec/x86/vp9dsp_init_10bpp.c:25:
./libavcodec/vp9dsp.h:123:50: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘ff_vp9_subpel_filters’
 extern const int16_t attribute_visibility_hidden ff_vp9_subpel_filters[3][16][8];
                                                  ^~~~~~~~~~~~~~~~~~~~~
ffbuild/common.mak:81: recipe for target 'libavcodec/x86/vp9dsp_init_12bpp.o' failed
make: *** [libavcodec/x86/vp9dsp_init_12bpp.o] Error 1
ffbuild/common.mak:81: recipe for target 'libavcodec/x86/vp9dsp_init_10bpp.o' failed
make: *** [libavcodec/x86/vp9dsp_init_10bpp.o] Error 1
In file included from libavcodec/x86/vp9dsp_init.c:26:0:
./libavcodec/vp9dsp.h:123:50: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘ff_vp9_subpel_filters’
 extern const int16_t attribute_visibility_hidden ff_vp9_subpel_filters[3][16][8];
                                                  ^~~~~~~~~~~~~~~~~~~~~
ffbuild/common.mak:81: recipe for target 'libavcodec/x86/vp9dsp_init.o' failed
make: *** [libavcodec/x86/vp9dsp_init.o] Error 1


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Dictatorship: All citizens are under surveillance, all their steps and
actions recorded, for the politicians to enforce control.
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