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

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Date:   Wed, 24 Jul 2019 09:05:33 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Wed, Jul 24, 2019 at 03:35:16PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 24, 2019 at 07:55:25AM -0500, Josh Poimboeuf wrote:
> > On Wed, Jul 24, 2019 at 09:47:32AM +0200, Peter Zijlstra wrote:
> > > 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?
> > 
> > According to the github issue[1] my patch doesn't fix the warning with
> > Clang.  So questions remain:
> 
> I was thinking your patch resulted in the warning due to the exception
> code gaining a ->func.

I had the same thought.

> But then that doesn't make sense either, because all that lives in
> copy_user_64.S which is a completely different translation unit.

Hm?  __copy_from_user() uses raw_copy_from_user() to do the STAC/CLAC in
a header file for the __builtin_constant_p() case.

> > a) what is objtool actually warning about?
> 
> CLAC with AC already clear. Either we do double CLAC at the end, or we
> do CLAC without having done STAC first.
> 
> The issue isn't BAD(tm), as AC clear is the safe state, but it typically
> indicates confused code flow.

But as I said my patch didn't fix the Clang warning.  Or is there
another redundant UACCESS disable you know about?

> > b) why doesn't objtool detect the case I found?
> 
> With GCC you mean? Yes, that is really really weird.

With both compilers...

> Let me go stare at objdump output for this file (which doesn't build
> with:
> 
>    make O=defconfig-build/ drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o
> )

-- 
Josh