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

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Date:   Sat, 15 Aug 2020 05:28:55 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, 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>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Sat, Aug 15, 2020 at 2:27 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Fri, Aug 14, 2020 at 3:57 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Fri, Aug 14, 2020 at 2:19 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > >
> > > On Fri, Aug 14, 2020 at 7:29 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > > >
> > > > Thanks for the proposal.
> > > >
> > > > I have adapted it to fit my patchset against Linux v5.8.
> > > >
> > > > Both Debian's GCC-10 and a snapshot version of LLVM toolchain
> > > > v11.0.0-rc1+ seems to be OK.
> > > >
> > >
> > > Yupp, OK.
> > >
> > > I was able to boot FreeDOS 1.2 VM in VirtualBox GUI.
> >
> > Hi Sedat,
> > Apologies, but it's not clear to me precisely which patch you tested.
> > Can you please confirm whether you tested:
> > 1. Arnd's patch that started this thread.
> > 2. My proposed diff adding -fno-addrsig to CFLAGS_powernow-k6.o.
> > 3. My proposed diff removing __force_order from the kernel.
> >
> > I'm hoping you were referring to testing 3., but it's not clear to me.
>
> Ah, sorry, I missed your comment on github:
> https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674282107
>
> Ok, I will look at more disassembly next week and hopefully have a
> patch ready, with your tested by tag.
>

Sorry for not being precise - I tested with solution (3.).
Later I added the diff I used as mentioned in your above comment.

See [1]:

> In a 2nd run building with a selfmade clang-11 and LLVM "bin"utils is fine, too.

I cannot say much to older versions of GCC and/or LLVM/Clang if
removing "__force_order" works fine.

Another (4.) solution:
Sami tried successfully by adding "__weak" declaration with
CONFIG_LKDTM=m (see [2]).
I am OK if this works, too.

Please, see my attachments.

- Sedat -

[1] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674340760
[2] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674182703

View attachment "dmesg-T_5.8.1-9-amd64-llvm11-ias.txt" of type "text/plain" (72921 bytes)

Download attachment "config-5.8.1-9-amd64-llvm11-ias" of type "application/octet-stream" (233291 bytes)