[llvm-dev] Demotion of shared symbols resolved from the dynamic linker.

[Fāng-ruì Sòng via llvm-dev]

On Fri, Feb 28, 2020 at 7:24 AM Sean Fertile via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On PowerPC we have a failing address sanitizer test when linking with LLD.
> The issue is that the symbol '__libc_stack_end' is resolved to an undefined
> weak symbol when linking with LLD but not when linking (with the exact same
> command/input) with other linkers. Tracing the symbol I see it is resolved
> to a definition in the dynamic linker as expected:
>
> /home/sfertile/LLVM_MonoRepo/build/lib/clang/11.0.0/lib/linux/libclang_rt.asan-powerpc64le.a(sanitizer_linux.cpp.o):
> reference to __libc_stack_end
> /lib/powerpc64le-linux-gnu/libpthread.so.0: reference to __libc_stack_end
> /lib/powerpc64le-linux-gnu/ld64.so.2: shared definition of __libc_stack_end
> <internal>: reference to __libc_stack_end
>
> The last line in the trace output shows where we demote the shared
> definition to an undefined symbol here:
> https://github.com/llvm/llvm-project/blob/c8bfed05e21f945b5ac71cd01d62e2854a8ddcf9/lld/ELF/Driver.cpp#L1505
>
> I'm guessing the fix is that if `needsInterpSection()` is true then the
> dynamic linker should be marked as needed. Its going to end up in the
> DT_NEEDED anyway so the symbols can't become dangling references. In my
> case then, the demotion won't happen and everything works as expected. Is
> this the right direction?
>

Can you name the target and upload a reproduce file (--reproduce=) ?

I have checked the x86-64 case: clang -fsanitize=address -fuse-ld=lld a.c
-o a -Wl,-y,__libc_stack_end
The demotion works as expected. ld.so is linked because of AS_NEEDED(...)
in libc.so (a linker script).
It is dropped from DT_NEEDED entries because it only provides definitions
resolving weak references.

--dynamic-linker= pointing to ld.so does not mean ld.so will be added to
DT_NEEDED.
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