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

Peter Zijlstra
2024-11-08T15:13:40Z

On Fri, Nov 08, 2024 at 07:57:16AM -0700, Nathan Chancellor wrote:
> 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@infradead.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?

I took the .config from the report you pointed me at yesterday.

https://lore.kernel.org/202411071743.HZsCuurm-lkp@intel.com/

And specifically the kvm build targets from:

$ make O=tmp-build/ LLVM=-19 arch/x86/kvm/

show this problem.

I just ran a defconfig, and that seems to behave properly. Notably,
vmlinux.o (definitely a link target) has entsize=8 for the relevant
section.

I'm not sure how the kvm targets might be 'special'.