linux-kernel - Re: x86 - clang / objtool status

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Message-ID: <20190724074732.GJ3402@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Jul 2019 09:47:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: x86 - clang / objtool status

On Tue, Jul 23, 2019 at 09:43:24PM -0500, Josh Poimboeuf wrote:
> On Thu, Jul 18, 2019 at 10:40:09PM +0200, Thomas Gleixner wrote:
> 
> >   drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x86: redundant UACCESS disable
> 
> Looking at this one, I think I agree with objtool.
> 
> PeterZ, Linus, I know y'all discussed this code a few months ago.
> 
> __copy_from_user() already does a CLAC in its error path.  So isn't the
> user_access_end() redundant for the __copy_from_user() error path?

Hmm, is this a result of your c705cecc8431 ("objtool: Track original function across branches") ?

I'm thinking it might've 'overlooked' the CLAC in the error path before
(because it didn't have a related function) and now it sees it and
worries about it.

Then again, I'm not seeing this warning on my GCC builds; so what's
happening?