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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 12 Jun 2019 16:12:29 +0200
Message-ID: <CACT4Y+a0H0NiMmydmw1qOA=zUXDmBZXHmh6-fp9nU0UtAPZvxQ@mail.gmail.com>
Subject: Re: [PATCH v3 2/3] x86: Use static_cpu_has in uaccess region to avoid instrumentation
To:     Marco Elver <elver@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Fri, Jun 7, 2019 at 11:44 AM Marco Elver <elver@google.com> wrote:
>
> Gentle ping.  I would appreciate quick feedback if this approach is reasonable.
>
> Peter: since you suggested that we should not change objtool, did you
> have a particular approach in mind that is maybe different from v2 and
> v3? Or is this what you were thinking of?
>
> Many thanks!
>
> On Fri, 31 May 2019 at 17:11, Marco Elver <elver@google.com> wrote:
> >
> > This patch is a pre-requisite for enabling KASAN bitops instrumentation;
> > using static_cpu_has instead of boot_cpu_has avoids instrumentation of
> > test_bit inside the uaccess region. With instrumentation, the KASAN
> > check would otherwise be flagged by objtool.
> >
> > For consistency, kernel/signal.c was changed to mirror this change,
> > however, is never instrumented with KASAN (currently unsupported under
> > x86 32bit).
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> > Suggested-by: H. Peter Anvin <hpa@zytor.com>
> > ---
> > Changes in v3:
> > * Use static_cpu_has instead of moving boot_cpu_has outside uaccess
> >   region.
> >
> > Changes in v2:
> > * Replaces patch: 'tools/objtool: add kasan_check_* to uaccess
> >   whitelist'
> > ---
> >  arch/x86/ia32/ia32_signal.c | 2 +-
> >  arch/x86/kernel/signal.c    | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> > index 629d1ee05599..1cee10091b9f 100644
> > --- a/arch/x86/ia32/ia32_signal.c
> > +++ b/arch/x86/ia32/ia32_signal.c
> > @@ -358,7 +358,7 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
> >                 put_user_ex(ptr_to_compat(&frame->uc), &frame->puc);
> >
> >                 /* Create the ucontext.  */
> > -               if (boot_cpu_has(X86_FEATURE_XSAVE))
> > +               if (static_cpu_has(X86_FEATURE_XSAVE))


Peter Z or A, does it look good to you? Could you please Ack this?


> >                         put_user_ex(UC_FP_XSTATE, &frame->uc.uc_flags);
> >                 else
> >                         put_user_ex(0, &frame->uc.uc_flags);
> > diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> > index 364813cea647..52eb1d551aed 100644
> > --- a/arch/x86/kernel/signal.c
> > +++ b/arch/x86/kernel/signal.c
> > @@ -391,7 +391,7 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
> >                 put_user_ex(&frame->uc, &frame->puc);
> >
> >                 /* Create the ucontext.  */
> > -               if (boot_cpu_has(X86_FEATURE_XSAVE))
> > +               if (static_cpu_has(X86_FEATURE_XSAVE))
> >                         put_user_ex(UC_FP_XSTATE, &frame->uc.uc_flags);
> >                 else
> >                         put_user_ex(0, &frame->uc.uc_flags);
> > --
> > 2.22.0.rc1.257.g3120a18244-goog
> >