linux-kernel - Re: [PATCH RFC 4/4] x86/srso: Use CALL-based return thunks to reduce overhead

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Date:   Wed, 13 Sep 2023 15:17:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Babu Moger <babu.moger@....com>, David.Kaplan@....com,
        Nikolay Borisov <nik.borisov@...e.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC 4/4] x86/srso: Use CALL-based return thunks to reduce
 overhead

On Mon, Aug 21, 2023 at 12:27:23PM +0100, Andrew Cooper wrote:

> 
>   vmlinux.o: warning: objtool: srso_fam17_return_thunk(): can't find starting instruction
> 

> @@ -288,26 +283,22 @@ SYM_START(srso_fam17_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
>   * and execution will continue at the return site read from the top of
>   * the stack.
>   */
> -SYM_INNER_LABEL(srso_fam17_safe_ret, SYM_L_GLOBAL)
> +SYM_INNER_LABEL(srso_fam17_return_thunk, SYM_L_GLOBAL)

This srso_safe_ret -> srso_fam17_safe_ret you forgot in the last patch,
is then here renamed yet again to srso_fam17_return_thunk.

And there is your objtool splat.

> +	UNWIND_HINT_FUNC
> +	ANNOTATE_NOENDBR
>  	lea 8(%_ASM_SP), %_ASM_SP
> +	ANNOTATE_UNRET_SAFE
>  	ret
>  	int3
>  	int3
>  	/* end of movabs */
>  	lfence
> -	call srso_fam17_safe_ret
> +	call srso_fam17_return_thunk
>  	ud2
> -SYM_CODE_END(srso_fam17_safe_ret)
> +SYM_CODE_END(srso_fam17_return_thunk)
>  SYM_FUNC_END(srso_fam17_untrain_ret)
>  __EXPORT_THUNK(srso_fam17_untrain_ret)
>