Re: [PATCH 3/5] objtool: Support Clang non-section symbols in ORC generation

Miroslav Benes
2020-04-03T08:58:24Z

On Wed, 1 Apr 2020, Josh Poimboeuf wrote:

> When compiling the kernel with AS=clang, objtool produces a lot of
> warnings:
>
> warning: objtool: missing symbol for section .text
> warning: objtool: missing symbol for section .init.text
> warning: objtool: missing symbol for section .ref.text
>
> It then fails to generate the ORC table.
>
> The problem is that objtool assumes text section symbols always exist.
> But the Clang assembler is aggressive about removing them.
>
> When generating relocations for the ORC table, objtool always tries to
> reference instructions by their section symbol offset. If the section
> symbol doesn't exist, it bails.
>
> Do a fallback: when a section symbol isn't available, reference a
> function symbol instead.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/669
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Reported-by: Dmitry Golovin <dima@golovin.in>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
> tools/objtool/orc_gen.c | 33 ++++++++++++++++++++++++++-------
> 1 file changed, 26 insertions(+), 7 deletions(-)
>
> diff --git a/tools/objtool/orc_gen.c b/tools/objtool/orc_gen.c
> index 41e4a2754da4..4c0dabd28000 100644
> --- a/tools/objtool/orc_gen.c
> +++ b/tools/objtool/orc_gen.c
> @@ -88,11 +88,6 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
> struct orc_entry *orc;
> struct rela *rela;
>
> - if (!insn_sec->sym) {
> - WARN("missing symbol for section %s", insn_sec->name);
> - return -1;
> - }
> -
> /* populate ORC data */
> orc = (struct orc_entry *)u_sec->data->d_buf + idx;
> memcpy(orc, o, sizeof(*orc));
> @@ -105,8 +100,32 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
> }
> memset(rela, 0, sizeof(*rela));
>
> - rela->sym = insn_sec->sym;
> - rela->addend = insn_off;
> + if (insn_sec->sym) {
> + rela->sym = insn_sec->sym;
> + rela->addend = insn_off;
> + } else {
> + /*
> + * The Clang assembler doesn't produce section symbols, so we
> + * have to reference the function symbol instead:
> + */
> + rela->sym = find_symbol_containing(insn_sec, insn_off);
> + if (!rela->sym) {
> + /*
> + * Hack alert. This happens when we need to reference
> + * the NOP pad insn immediately after the function.
> + */
> + rela->sym = find_symbol_containing(insn_sec,
> + insn_off - 1);
> + }

I suppose there is always just one NOP pad insn, right? Anyway, it would
be better to get rid of it as you proposed.

> + if (!rela->sym) {
> + WARN("missing symbol for insn at offset 0x%lx\n",
> + insn_off);
> + return -1;
> + }
> +
> + rela->addend = insn_off - rela->sym->offset;
> + }
> +
> rela->type = R_X86_64_PC32;
> rela->offset = idx * sizeof(int);
> rela->sec = ip_relasec;

Miroslav