On Thu, Mar 18, 2021 at 06:11:17PM +0100, Peter Zijlstra wrote:
> When the compiler emits: "CALL __x86_indirect_thunk_\reg" for an
> indirect call, have objtool rewrite it to:
>
> ALTERNATIVE "call __x86_indirect_thunk_\reg",
> "call *%reg", ALT_NOT(X86_FEATURE_RETPOLINE)
>
> Additionally, in order to not emit endless identical
> .altinst_replacement chunks, use a global symbol for them, see
> __x86_indirect_alt_*.
>
> This also avoids objtool from having to do code generation.
>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
This is better than I expected. Nice workaround for not generating
code.
> +.macro ALT_THUNK reg
> +
> + .align 1
> +
> +SYM_FUNC_START_NOALIGN(__x86_indirect_alt_call_\reg)
> + ANNOTATE_RETPOLINE_SAFE
> +1: call *%\reg
> +2: .skip 5-(2b-1b), 0x90
> +SYM_FUNC_END(__x86_indirect_alt_call_\reg)
> +
> +SYM_FUNC_START_NOALIGN(__x86_indirect_alt_jmp_\reg)
> + ANNOTATE_RETPOLINE_SAFE
> +1: jmp *%\reg
> +2: .skip 5-(2b-1b), 0x90
> +SYM_FUNC_END(__x86_indirect_alt_jmp_\reg)
This mysterious code needs a comment. Shouldn't it be in
.altinstr_replacement or something?
Also doesn't the alternative code already insert nops?
> +int arch_rewrite_retpoline(struct objtool_file *file,
> + struct instruction *insn,
> + struct reloc *reloc)
> +{
> + struct symbol *sym;
> + char name[32] = "";
> +
> + if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
> + return 0;
> +
> + sprintf(name, "__x86_indirect_alt_%s_%s",
> + insn->type == INSN_JUMP_DYNAMIC ? "jmp" : "call",
> + reloc->sym->name + 21);
> +
> + sym = find_symbol_by_name(file->elf, name);
> + if (!sym) {
> + sym = elf_create_undef_symbol(file->elf, name);
> + if (!sym) {
> + WARN("elf_create_undef_symbol");
> + return -1;
> + }
> + }
> +
> + elf_add_alternative(file->elf, insn, sym,
> + ALT_NOT(X86_FEATURE_RETPOLINE), 5, 5);
> +
> + return 0;
> +}
Need to propagate the error.
--
Josh
