linux-kernel - Re: [PATCH] x86: work around clang IAS bug referencing __force

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Date:   Fri, 21 Aug 2020 23:55:52 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Will Deacon <will@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Sat, Aug 22, 2020 at 02:43:08AM +0200, Thomas Gleixner wrote:
> On Fri, Aug 21 2020 at 16:16, Nick Desaulniers wrote:
> > On Fri, Aug 21, 2020 at 4:04 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >> On Fri, Aug 21, 2020 at 02:37:48AM +0200, Thomas Gleixner wrote:
> >> The gcc bug I linked to earlier is only fixed in gcc-6 onwards. Is that
> >
> > (based on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602#c14)
> >
> >> good enough to remove force_order? I can test gcc-4.9 and gcc-5 to check
> >> if it would currently have any impact.
> 
> And that test tells you what exactly? That your particular build of
> those compilers does not have the problem. A truly scientific approach.

More that the current kernel code doesn't have that problem, but yeah,
it might creep in later.

> 
> > I think checking the disassemblies with a pre-gcc-6 would be good
> > enough then; that bug isn't specific to this particular case.
> 
> What? I clearly want a statement from the GCC people that this won't
> happen on pre gcc6 compilers and not just some 'works for me' statement
> based on a randomly picked compiler build.

Presumably also from clang that the compiler does have protections
against this, as opposed to doesn't happen today.

> 
> Thanks,
> 
>         tglx

Cc Segher.

Segher, we were looking at gcc PR82602, where IRA could reorder volatile
asm's (reported on ARM). The fix was backported to gcc-6. Do you know if
there is any reason the problem couldn't occur on x86 on older gcc
without the fix?

Thanks.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602