On Mon, 11 Jul 2022, Martin Storsjö wrote:
Hi,
Thanks for your patch! While the patch probably is worthwhile to pursue,
ffmpeg does work on Android 6 and newer (with the aarch64 assembly), so
there's some gaps/misses in the reasoning in the patch description.
On Sun, 10 Jul 2022, Triang3l wrote:
To support PIC, all AArch64 assembly code in FFmpeg uses the `movrel` macro
to obtain addresses of labels, such as lookup table constants, in a way
that with CONFIG_PIC being 1, PC-relative addresses of labels are computed
via the `adrp` instruction.
This approach, however, is suitable only for labels defined in the same
object file. For `adrp` to work directly between object files, the linker
has to perform a text relocation.
No, it doesn't. It's fully possible to build such libraries without text
relocations currently.
My memory of the situation was that we're linking our shared libraries with
-Wl,-Bsymbolic, which means that those symbol references are bound at link
time, so the offset from adrp to the target symbol is fixed at link time, and
no text relocation is needed.
However I did try to link such shared libraries, removing the -Wl,-Bsymbolic
argument, and it still succeeds. So I'm a bit unsure what really makes it
work for me when it isn't working for you.
Andreas Rheinhardt kindly reminded me that when linking libavcodec.so, we
add -Wl,--version-script,libavcodec/libavcodec.ver, which makes e.g.
ff_cos_32 a non-exported symbol, which means that it can't be interposed,
and thus the relocation can be fixed at link time.
If I remove that argument, I can reproduce the linker errors, and adding
-Wl,-Bsymbolic fixes it.
I wonder if we could mark these symbols as ELF hidden, so that they
wouldn't need to be interposable at all, even when you link the static
library into a separate shared library?
// Martin
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