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

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Message-ID: <CA+icZUWGHo3mkN4=ZYeU37p8KJhxTXS5QKcAvq_HaqTHmFkw2g@mail.gmail.com>
Date:   Sat, 22 Aug 2020 11:51:56 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        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>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Sat, Aug 22, 2020 at 11:23 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> On Sat, Aug 22, 2020 at 10:42 AM Segher Boessenkool
> <segher@...nel.crashing.org> wrote:
> >
> > 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.
> >
>
> [1] says:
>
> Current Minimal Requirements
> ...
> ====================== ===============  ========================================
>         Program        Minimal version       Command to check the version
> ====================== ===============  ========================================
> GNU C                  4.9              gcc --version
>
> - Sedat -
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/changes.rst#n32

[ CC Miguel Ojeda (Compiler Attributes maintainer) ]

There exist gcc-4.8 and gcc-4.9 for Debian/jessie where EOL was June
30, 2020 (see [1] and [2]).

In the latest available version "4.9.2-10+deb8u1" I see no PR82602 was
backported (see [3] and [4]).

I am asking myself who is using such ancient compilers?
Recently, I threw away GCC-8 from my Debian system.

If this is a real problem with GCC version <= 5, so can this be moved
to a GCC specific include header-file?
Thinking of include/linux/compiler-gcc.h or
include/linux/compiler_types.h with a GCC-VERSION check?

Thoughts?

- Sedat -

P.S.: Yesterday, I built with dropping __force_order entirely and LLVM
toolchain v11.0.0-rc2 on Debian/unstable AMD64 on top of recent Linux
v5.9-rc1+.

[1] https://packages.debian.org/search?keywords=gcc-4
[2] https://wiki.debian.org/LTS
[3] https://sources.debian.org/src/gcc-4.9/
[4] https://sources.debian.org/src/gcc-4.9/4.9.2-10+deb8u1/debian/patches/