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

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Message-ID: <naxuqdzhtcbtq7jdc3wiee2fawjzozhhfiztldin7bnl2mhk4h@eimhmkrc6kgs>
Date: Fri, 14 Mar 2025 12:52:32 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.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 08:38:48PM +0100, Peter Zijlstra wrote:
> 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.

That's at least enforced by kconfig dependencies as
MITIGATION_UNRET_ENTRY depends on MITIGATION_RETHUNK.

If you wanted to enforce it on the objtool level, we might need to do
some makefile acrobatics to defer --rethunk to vmlinux link time for the
above mentioned case.

-- 
Josh