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

From: Arnd Bergmann <arnd@arndb.de>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	Ilie Halip <ilie.halip@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9
Date: Wed, 28 Aug 2019 17:13:59 +0200	[thread overview]
Message-ID: <CAK8P3a0bY9QfamCveE3P4H+Nrs1e6CTqWVgiY+MCd9hJmgMQZg@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a0nJL+3hxR0U9kT_9Y4E86tofkOnVzNTEvAkhOFxOEA3Q@mail.gmail.com>

On Wed, Aug 28, 2019 at 11:00 AM Arnd Bergmann <arnd@arndb.de> wrote:
> On Tue, Aug 27, 2019 at 11:22 PM 'Nick Desaulniers' via Clang Built Linux <clang-built-linux@googlegroups.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