Re: The trouble with __weak and objtool got worse

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Apr 15, 2022 at 11:21:34AM -0700, Josh Poimboeuf wrote:
> On Fri, Apr 15, 2022 at 10:40:22AM -0700, Nick Desaulniers wrote:
> > On Fri, Apr 15, 2022 at 8:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Fri, Apr 15, 2022 at 01:19:04PM +0200, Peter Zijlstra wrote:
> > > > Alternatively:
> > > >
> > > >   https://sourceware.org/pipermail/binutils/2020-December/114671.html
> > > >
> > > > seems to suggest: -Wa,--generate-unused-section-symbols=yes, ought to
> > > > work, except I'm getting:
> > > >
> > > > $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,--generate-unused-section-symbols=yes -c foo*.c
> > > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > >
> > > Reading so hard...
> > >
> > >  $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,-generate-unused-section-symbols=yes -c foo*.c
> > >
> > > seems to actually work.
> > 
> > $ clang hello.c -Wa,-generate-unused-section-symbols=yes
> > clang-15: error: unsupported argument
> > '-generate-unused-section-symbols=yes' to option '-Wa,'
> > 
> > :)
> > 
> > I recall LLVM not emitting STT_SECTION symbols being problematic for
> > objtool in the past.
> > Old pre-lore archive:
> > https://groups.google.com/g/clang-built-linux/c/1C6YoJKBsQQ/m/a8IS1NjGAgAJ
> > via https://github.com/ClangBuiltLinux/linux/issues/1209.
> > (It doesn't look too bad for me to implement)
> 
> Right, and that objtool "fix" -- silently falling back to
> non-section-symbol relocations instead of erroring out -- is causing
> this issue, when it's done for weak symbols.
> 
> If LLVM assembler doesn't support this option then we may have to go
> with something like this?  I can't seem to recreate so I'm not able to
> test.
> 
> We'd also need some objtool checks to make sure the non-section-symbol
> fallback isn't being done for a weak symbol.  Or just remove the
> fallback altogether and force section symbols whereever they're needed,
> similar to the below (untested).

(If the patch works, we could hopefully come up with something better,
using asm goto, that doesn't involve actual instructions)

-- 
Josh