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

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Date:   Sat, 22 Aug 2020 03:41:33 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "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

Hi Arvind,

On Fri, Aug 21, 2020 at 11:55:52PM -0400, Arvind Sankar wrote:
> 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.

I know ;-)

> Do you know if
> there is any reason the problem couldn't occur on x86 on older gcc
> without the fix?

No, I see no particular reason, at least GCC 5 seems vulnerable.  (The
GCC 5 release branch was closed at the time this bug report was made,
already).  There is no reason I see why it would work on x86 but fail
elsewhere, either.


Segher