linux-kernel - Re: [PATCH v2 11/12] retpoline/objtool: Disable some objtool warnings

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Date:   Thu, 4 Jan 2018 08:38:14 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     tglx@...utronix.de, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v2 11/12] retpoline/objtool: Disable some objtool warnings

On Wed, Jan 03, 2018 at 06:00:18PM -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> With the indirect call thunk enabled compiler two objtool
> warnings are triggered very frequently and make the build
> very noisy.
> 
> I don't see a good way to avoid them, so just disable them
> for now.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
>
> ---
>  tools/objtool/check.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 9b341584eb1b..435c71f944dc 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -503,8 +503,13 @@ static int add_call_destinations(struct objtool_file *file)
>  			insn->call_dest = find_symbol_by_offset(insn->sec,
>  								dest_off);
>  			if (!insn->call_dest) {
> +#if 0
> +				/* Compilers with -mindirect-branch=thunk-extern trigger
> +				 * this everywhere on x86. Disable for now.
> +				 */
>  				WARN_FUNC("can't find call dest symbol at offset 0x%lx",
>  					  insn->sec, insn->offset, dest_off);
> +#endif
>  				return -1;
>  			}
>  		} else if (rela->sym->type == STT_SECTION) {
> @@ -1716,8 +1721,14 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
>  					return 1;
>  
>  			} else if (func && has_modified_stack_frame(&state)) {
> +#if 0
> +				/* Compilers with -mindirect-branch=thunk-extern trigger
> +				 * this everywhere on x86. Disable for now.
> +				 */
> +
>  				WARN_FUNC("sibling call from callable instruction with modified stack frame",
>  					  sec, insn->offset);
> +#endif
>  				return 1;
>  			}
>  
> -- 
> 2.14.3

NAK.  We can't blindly disable objtool warnings, that will break
livepatch and the ORC unwinder.  If you share a .o file (or the GCC
code) I can look at adding retpoline support.

-- 
Josh