linux-kernel - Re: [PATCH 06/13] objtool: Remove --unret dependency on --rethunk

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Message-ID: <20250314193848.GE36322@noisy.programming.kicks-ass.net>
Date: Fri, 14 Mar 2025 20:38:48 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Brendan Jackman <jackmanb@...gle.com>,
	Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH 06/13] objtool: Remove --unret dependency on --rethunk

On Fri, Mar 14, 2025 at 12:29:04PM -0700, Josh Poimboeuf wrote:
> With unret validation enabled and IBT/LTO disabled, objtool runs on TUs
> with --rethunk and on vmlinux.o with --unret.  So this dependency isn't
> valid as they don't always run on the same object.
> 
> This error never triggered before because --unret is always coupled with
> --noinstr, so the first conditional in opts_valid() returns early due to
> opts.noinstr being true.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  tools/objtool/builtin-check.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
> index 387d56a7f5fb..c7275cf7641b 100644
> --- a/tools/objtool/builtin-check.c
> +++ b/tools/objtool/builtin-check.c
> @@ -151,11 +151,6 @@ static bool opts_valid(void)
>  		return true;
>  	}
>  
> -	if (opts.unret && !opts.rethunk) {
> -		ERROR("--unret requires --rethunk");
> -		return false;
> -	}

But but but, the whole UNTRAIN_RET stuff relies on return thunks. Even
though objtool can do that VALUDATE_UNRET_BEGIN -> VALIDATE_UNRET_END
check without there being return thunks, it simply doesn't make sense.