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

["Martin Storsjö" <martin@martin.st>]

On Thu, 14 Jul 2022, Michael Niedermayer wrote:

> 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

Thanks, it seems I need to add an explicit include of libavutil/internal.h 
in vp9dsp.h. Fixing.

// Martin
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