linux-kernel - Re: [PATCH 01/11] objtool: Generic annotation infrastructure

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Message-ID: <20241108145716.GA2564051@thelio-3990X>
Date: Fri, 8 Nov 2024 07:57:16 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>, Fangrui Song <i@...kray.me>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	x86@...nel.org, kvm@...r.kernel.org, ndesaulniers@...gle.com,
	morbo@...gle.com, justinstitt@...gle.com, llvm@...ts.linux.dev
Subject: Re: [PATCH 01/11] objtool: Generic annotation infrastructure

On Fri, Nov 08, 2024 at 03:16:00PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 04, 2023 at 10:37:03AM +0100, Peter Zijlstra wrote:
> > Avoid endless .discard.foo sections for each annotation, create a
> > single .discard.annotate section that takes an annotation type along
> > with the instruction.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> > --- a/include/linux/objtool.h
> > +++ b/include/linux/objtool.h
> > @@ -57,6 +57,13 @@
> >  	".long 998b\n\t"						\
> >  	".popsection\n\t"
> >  
> > +#define ASM_ANNOTATE(x)						\
> > +	"911:\n\t"						\
> > +	".pushsection .discard.annotate,\"M\",@progbits,8\n\t"	\
> > +	".long 911b - .\n\t"					\
> > +	".long " __stringify(x) "\n\t"				\
> > +	".popsection\n\t"
> > +
> >  #else /* __ASSEMBLY__ */
> >  
> >  /*
> > @@ -146,6 +153,14 @@
> >  	.popsection
> >  .endm
> >  
> > +.macro ANNOTATE type:req
> > +.Lhere_\@:
> > +	.pushsection .discard.annotate,"M",@progbits,8
> > +	.long	.Lhere_\@ - .
> > +	.long	\type
> > +	.popsection
> > +.endm
> > +
> >  #endif /* __ASSEMBLY__ */
> >  
> >  #else /* !CONFIG_OBJTOOL */
> > @@ -167,6 +182,8 @@
> >  .endm
> >  .macro REACHABLE
> >  .endm
> > +.macro ANNOTATE
> > +.endm
> >  #endif
> >  
> >  #endif /* CONFIG_OBJTOOL */
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -2308,6 +2308,41 @@ static int read_unwind_hints(struct objt
> >  	return 0;
> >  }
> >  
> > +static int read_annotate(struct objtool_file *file, void (*func)(int type, struct instruction *insn))
> > +{
> > +	struct section *rsec, *sec;
> > +	struct instruction *insn;
> > +	struct reloc *reloc;
> > +	int type;
> > +
> > +	rsec = find_section_by_name(file->elf, ".rela.discard.annotate");
> > +	if (!rsec)
> > +		return 0;
> > +
> > +	sec = find_section_by_name(file->elf, ".discard.annotate");
> > +	if (!sec)
> > +		return 0;
> > +
> > +	for_each_reloc(rsec, reloc) {
> > +		insn = find_insn(file, reloc->sym->sec,
> > +				 reloc->sym->offset + reloc_addend(reloc));
> > +		if (!insn) {
> > +			WARN("bad .discard.annotate entry: %d", reloc_idx(reloc));
> > +			return -1;
> > +		}
> > +
> > +		type = *(u32 *)(sec->data->d_buf + (reloc_idx(reloc) * sec->sh.sh_entsize) + 4);
> > +
> > +		func(type, insn);
> > +	}
> > +
> > +	return 0;
> > +}
> 
> So... ld.lld hates this :-(
> 
> From an LLVM=-19 build we can see that:
> 
> $ readelf -WS tmp-build/arch/x86/kvm/vmx/vmenter.o | grep annotate
>   [13] .discard.annotate PROGBITS        0000000000000000 00028c 000018 08   M  0   0  1
> 
> $ readelf -WS tmp-build/arch/x86/kvm/kvm-intel.o | grep annotate
>   [ 3] .discard.annotate PROGBITS        0000000000000000 069fe0 0089d0 00   M  0   0  1
> 
> Which tells us that the translation unit itself has a sh_entsize of 8,
> while the linked object has sh_entsize of 0.
> 
> This then completely messes up the indexing objtool does, which relies
> on it being a sane number.
> 
> GCC/binutils very much does not do this, it retains the 8.
> 
> Dear clang folks, help?

Perhaps Fangrui has immediate thoughts, since this appears to be an
ld.lld thing? Otherwise, I will see if I can dig into this in the next
couple of weeks (I have an LF webinar on Wednesday that I am still
prepping for). Is this reproducible with just defconfig or something
else?

Cheers,
Nathan