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

From: Arnd Bergmann
Date: Wed Aug 28 2019 - 11:14:18 EST

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

> 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?