linux-kernel - Re: objtool warning "uses BP as a scratch register" with clang-9

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Date:   Wed, 28 Aug 2019 17:13:59 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9

On Wed, Aug 28, 2019 at 11:00 AM Arnd Bergmann <arnd@...db.de> wrote:
> On Tue, Aug 27, 2019 at 11:22 PM 'Nick Desaulniers' via Clang Built Linux <clang-built-linux@...glegroups.com> wrote:
I figured this one out as well:

> http://paste.ubuntu.com/p/XjdDsypRxX/
> 0x5BA1B7A1:arch/x86/ia32/ia32_signal.o: warning: objtool:
> ia32_setup_rt_frame()+0x238: call to memset() with UACCESS enabled
> 0x5BA1B7A1:arch/x86/kernel/signal.o: warning: objtool:
> __setup_rt_frame()+0x5b8: call to memset() with UACCESS enabled

When CONFIG_KASAN is set, clang decides to use memset() to set
the first two struct members in this function:

 static inline void sas_ss_reset(struct task_struct *p)
 {
        p->sas_ss_sp = 0;
        p->sas_ss_size = 0;
        p->sas_ss_flags = SS_DISABLE;
 }

and that is called from save_altstack_ex(). Adding a barrier() after
the sas_ss_sp() works around the issue, but is certainly not the
best solution. Any other ideas?

       Arnd