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Date:   Sat, 22 Aug 2020 03:41:33 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        Borislav Petkov <bp@alien8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juergen Gross <jgross@suse.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Will Deacon <will@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order
Message-ID: <20200822084133.GL28786@gate.crashing.org>
References: <87h7t6tpye.fsf@nanos.tec.linutronix.de> <20200813173701.GC4295@paulmck-ThinkPad-P72> <20200813180933.GA532283@rani.riverdale.lan> <875z9dioll.fsf@nanos.tec.linutronix.de> <20200820130641.GA536306@rani.riverdale.lan> <87zh6ohm03.fsf@nanos.tec.linutronix.de> <20200821230435.GA56974@rani.riverdale.lan> <CAKwvOdkoB+fT9tt7vgg1R6J-NEr77EWP5X8nFat_L-HvEzWGzA@mail.gmail.com> <87eenzqzmr.fsf@nanos.tec.linutronix.de> <20200822035552.GA104886@rani.riverdale.lan>
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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