Re: [PATCH 2/3] Revert "[AArch64][BZ #17711] Fix extern protected data handling"

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Fangrui,

>>Should -fno-semantic-interposition imply -fvisibility=protected
>>-mno-direct-extern-access?
>
> Unfortunately, no.  -fvisibility=protected imposes stricter requirement
> than -fno-semantic-interposition.  See
> https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected#protected-function-symbols-and-canonical-plt-entries
>
>   __attribute__((visibility("protected"))) void *foo() {
>     return (void *)foo;
>  }
>
> GNU ld does not support this on several architectures, including the common x86-32/x86-64/aarch64.

So you're saying this should only give an error if the executable takes the address of foo
without a GOT indirection? In principle compilers could use a GOT indirection when taking
the address of function symbols to make this work (either in the executable or for protected
function symbols in shared objects).

Note in the executable case, the linker can relax the GOT relocations so it would have almost
zero overhead.

Wilco