Re: [PATCH 1/5] objtool: Fix CONFIG_UBSAN_TRAP unreachable warnings

Kees Cook
2020-04-02T07:30:43Z

On Wed, Apr 01, 2020 at 01:23:25PM -0500, Josh Poimboeuf wrote:
> CONFIG_UBSAN_TRAP causes GCC to emit a UD2 whenever it encounters an
> unreachable code path. This includes __builtin_unreachable(). Because
> the BUG() macro uses __builtin_unreachable() after it emits its own UD2,
> this results in a double UD2. In this case objtool rightfully detects
> that the second UD2 is unreachable:
>
> init/main.o: warning: objtool: repair_env_string()+0x1c8: unreachable instruction
>
> We weren't able to figure out a way to get rid of the double UD2s, so
> just silence the warning.
>
> Cc: Kees Cook <keescook@chromium.org>
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

Thanks for finding a way to work around this!

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> tools/objtool/check.c | 17 +++++++++++++++--
> 1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index e3bb76358148..aaec5e1277ea 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -2382,14 +2382,27 @@ static bool ignore_unreachable_insn(struct instruction *insn)
> !strcmp(insn->sec->name, ".altinstr_aux"))
> return true;
>
> + if (!insn->func)
> + return false;
> +
> + /*
> + * CONFIG_UBSAN_TRAP inserts a UD2 when it sees
> + * __builtin_unreachable(). The BUG() macro has an unreachable() after
> + * the UD2, which causes GCC's undefined trap logic to emit another UD2
> + * (or occasionally a JMP to UD2).
> + */
> + if (list_prev_entry(insn, list)->dead_end &&
> + (insn->type == INSN_BUG ||
> + (insn->type == INSN_JUMP_UNCONDITIONAL &&
> + insn->jump_dest && insn->jump_dest->type == INSN_BUG)))
> + return true;
> +
> /*
> * Check if this (or a subsequent) instruction is related to
> * CONFIG_UBSAN or CONFIG_KASAN.
> *
> * End the search at 5 instructions to avoid going into the weeds.
> */
> - if (!insn->func)
> - return false;
> for (i = 0; i < 5; i++) {
>
> if (is_kasan_insn(insn) || is_ubsan_insn(insn))
> --
> 2.21.1
>

--
Kees Cook