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Date:   Fri, 11 Aug 2023 09:01:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, David.Kaplan@amd.com,
        Andrew.Cooper3@citrix.com, jpoimboe@kernel.org,
        gregkh@linuxfoundation.org, matz@suse.de,
        Nick Desaulniers <ndesaulniers@google.com>,
        joao.moreira@intel.com, samitolvanen@google.com
Subject: Re: [RFC][PATCH 02/17] x86/cpu: Clean up SRSO return thunk mess
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On Thu, Aug 10, 2023 at 02:37:56PM +0200, Peter Zijlstra wrote:

> After this patch things look equivalent to:
> 
> SYM_FUNC_START(foo)
> 	...
> 	ALTERNATIVE "ret; int3"
> 		    "jmp __x86_return_thunk", X86_FEATURE_RETHUNK
> 		    "jmp srso_return_thunk, X86_FEATURE_SRSO
> 		    "jmp srsi_alias_return_thunk", X86_FEATURE_SRSO_ALIAS
> SYM_FUNC_END(foo)
> 
> SYM_CODE_START(srso_return_thunk)
> 	UNWIND_HINT_FUNC
> 	ANNOTATE_NOENDBR
> 	call srso_safe_ret;
> 	ud2
> SYM_CODE_END(srso_return_thunk)
> 
> SYM_CODE_START(srso_alias_return_thunk)
> 	UNWIND_HINT_FUNC
> 	ANNOTATE_NOENDBR
> 	call srso_alias_safe_ret;
> 	ud2
> SYM_CODE_END(srso_alias_return_thunk)
> 

So it looks like the compilers are still not emitting int3 after jmp,
even with the SLS options enabled :/

This means the tail end of functions compiled with:

  -mharden-sls=all -mfunction-return=thunk-extern

Is still a regular: jmp __x86_return_thunk, no trailing trap.

  https://godbolt.org/z/Ecqv76YbE

If we all could please finally fix that, then I can rewrite the above to
effectively be:

SYM_FUNC_START(foo)
	...
	ALTERNATIVE "ret; int3"
		    "jmp __x86_return_thunk", X86_FEATURE_RETHUNK
		    "call srso_safe_ret, X86_FEATURE_SRSO
		    "call srso_alias_safe_ret", X86_FEATURE_SRSO_ALIAS
	int3 //  <--- *MISSING*
SYM_FUNC_END(foo)

Bonus points if I can compile time tell if a compiler DTRT, feature flag
or what have you in the preprocessor would be awesome.