Re: [PATCH v5 01/15] objtool: Add CONFIG_CFI_CLANG support

Peter Zijlstra
2021-10-14T10:23:45Z

On Wed, Oct 13, 2021 at 11:16:44AM -0700, Sami Tolvanen wrote:
> diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> index 1f2ae708b223..5fe31523e51f 100644
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -63,6 +63,23 @@ bool arch_callee_saved_reg(unsigned char reg)
> }
> }
>
> +unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc)
> +{
> + if (!reloc->addend)
> + return 0;
> +
> + if (reloc->type == R_X86_64_PC32 || reloc->type == R_X86_64_PLT32)
> + return reloc->addend + 4;
> +
> + return reloc->addend;
> +}
> +
> +unsigned long arch_cfi_jump_reloc_offset(unsigned long offset)
> +{
> + /* offset to the relocation in a jmp instruction */
> + return offset + 1;
> +}
> +
> unsigned long arch_dest_reloc_offset(int addend)
> {
> return addend + 4;
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index b18f0055b50b..cd09c93c34fb 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c

> @@ -575,6 +580,49 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi
> return 0;
> }
>
> +/*
> + * CONFIG_CFI_CLANG replaces function relocations to refer to an intermediate
> + * jump table. Undo the conversion so objtool can make sense of things.
> + */
> +static int fix_cfi_relocs(const struct elf *elf)
> +{
> + struct section *sec;
> + struct reloc *reloc;
> +
> + list_for_each_entry(sec, &elf->sections, list) {
> + list_for_each_entry(reloc, &sec->reloc_list, list) {
> + struct reloc *cfi_reloc;
> + unsigned long offset;
> +
> + if (!reloc->sym->sec->cfi_jt)
> + continue;
> +
> + if (reloc->sym->type == STT_SECTION)
> + offset = arch_cfi_section_reloc_offset(reloc);
> + else
> + offset = reloc->sym->offset;
> +
> + /*
> + * The jump table immediately jumps to the actual function,
> + * so look up the relocation there.
> + */
> + offset = arch_cfi_jump_reloc_offset(offset);
> + cfi_reloc = find_reloc_by_dest(elf, reloc->sym->sec, offset);
> +
> + if (!cfi_reloc || !cfi_reloc->sym) {
> + WARN("can't find a CFI jump table relocation at %s+0x%lx",
> + reloc->sym->sec->name, offset);
> + return -1;
> + }
> +
> + reloc->sym = cfi_reloc->sym;
> + reloc->addend = 0;
> + }
> + }
> +
> + return 0;
> +}

If that section ever gets modified and we end up running
elf_rebuild_reloc_section() on it, we're in trouble, right?

Do we want a fatal error in elf_rebuild_reloc_section() when ->cfi_jt is
set?