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

[Kees Cook <keescook@chromium.org>]

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