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

[Josh Poimboeuf]

On Mon, Oct 07, 2024 at 10:32:12AM +0200, Peter Zijlstra wrote:
> Replace the funny __warn_thunk thing with a more regular
> WARN_ON_ONCE(), and simplify the ifdeffery.
> 
> Notably this avoids RET from having recursive RETs (once from the
> thunk and once from the C function) -- recursive RET makes my head
> hurt for no good reason.

This could use an explanation for why the ifdefs can be removed and why
the alternative can be removed.

> +#define WARN_ONCE							\
> +	1: ALTERNATIVE "", "ud2", X86_FEATURE_ALWAYS ;			\
> +	ASM_BUGTABLE_FLAGS(1b, 0, 0, BUGFLAG_WARNING | BUGFLAG_ONCE) ;	\
> +	REACHABLE

Can we not use __FILE__ and __LINE__ here?  Also why not put this in
asm/bug.h?

>  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

Isn't this return thunk used before apply_returns()?  How does that not
trigger the warning?

-- 
Josh