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

Peter Zijlstra
2024-11-08T14:16:03Z

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?