linux-kernel - Re: [RFC][PATCH 2/2] x86: Clean up default rethunk warning

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Message-ID: <20241104114728.GTZyi0UHYKx-ZHL4kh@fat_crate.local>
Date: Mon, 4 Nov 2024 12:47:28 +0100
From: Borislav Petkov <bp@...en8.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: david.kaplan@....com, jpoimboe@...hat.com, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [RFC][PATCH 2/2] x86: Clean up default rethunk warning

On Mon, Oct 07, 2024 at 10:32:12AM +0200, Peter Zijlstra wrote:
> -	.section .text..__x86.indirect_thunk
> +#define WARN_ONCE							\

This should be in the asm section of arch/x86/include/asm/bug.h so that other
asm code can use it. It will come in handy...

> +	1: ALTERNATIVE "", "ud2", X86_FEATURE_ALWAYS ;			\

... but uff, you can't because of this ALTERNATIVE. This is a conditional
WARN_ONCE.  Yuck.

I guess ALT_WARN_ONCE or so...

> +	ASM_BUGTABLE_FLAGS(1b, 0, 0, BUGFLAG_WARNING | BUGFLAG_ONCE) ;	\
> +	REACHABLE
>  
> +	.section .text..__x86.indirect_thunk
>  
>  .macro POLINE reg
>  	ANNOTATE_INTRA_FUNCTION_CALL
> @@ -382,16 +387,15 @@ SYM_FUNC_END(call_depth_return_thunk)
>  SYM_CODE_START(__x86_return_thunk)
>  	UNWIND_HINT_FUNC
>  	ANNOTATE_NOENDBR
> -#if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
> -    defined(CONFIG_MITIGATION_SRSO) || \
> -    defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
> -	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
> -		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
> -#else
> +
> +#ifdef CONFIG_X86_64
> +	WARN_ONCE
> +#endif

And you can add an empty 32-bit WARN_ONCE macro so that we don't have this
ifdeffery here where ifdeffery gives the last drop of making this file totally
unreadable...

-- 
Regards/Gruss,
    Boris.

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