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

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Message-ID: <20190829232439.w3whzmci2vqtq53s@treble>
Date:   Thu, 29 Aug 2019 18:24:39 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.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 05:40:01PM +0200, Arnd Bergmann wrote:
> On Wed, Aug 28, 2019 at 5:28 PM Arnd Bergmann <arnd@...db.de> wrote:
> > On Wed, Aug 28, 2019 at 5:22 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > > On Wed, Aug 28, 2019 at 05:13:59PM +0200, Arnd Bergmann wrote:
> > > >
> > > > 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?
> > >
> > > Wow, is the compiler allowed to insert memset calls like that?  Seems a
> > > bit overbearing, at least in a kernel context.  I don't recall GCC ever
> > > doing it.
> >
> > Yes, it's free to assume that any standard library function behaves
> > as defined, so it can and will turn struct assignments into memcpy
> > or back, or replace string operations with others depending on what
> > seems better for optimization.
> >
> > clang is more aggressive than gcc here, and this has caused some
> > other problems in the past, but it's usually harmless.
> >
> > In theory, we could pass -ffreestanding to tell the compiler
> > not to make assumptions about standard library function behavior,
> > but that turns off all kinds of useful optimizations. The problem
> > is really that the kernel is neither exactly hosted nor freestanding.
> 
> A slightly better workaround is to move the sas_ss_reset() out of
> the try/catch block. Not sure if this is safe.
> 
>       Arnd
> 
> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> index 1cee10091b9f..14f8decf0ebc 100644
> --- a/arch/x86/ia32/ia32_signal.c
> +++ b/arch/x86/ia32/ia32_signal.c
> @@ -379,6 +379,9 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
>                 put_user_ex(*((u64 *)&code), (u64 __user *)frame->retcode);
>         } put_user_catch(err);
> 
> +       if (current->sas_ss_flags & SS_AUTODISARM)
> +               sas_ss_reset(current);
> +
>         err |= __copy_siginfo_to_user32(&frame->info, &ksig->info, false);
>         err |= ia32_setup_sigcontext(&frame->uc.uc_mcontext, fpstate,
>                                      regs, set->sig[0]);
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 8eb7193e158d..fd49d28abbc5 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -414,6 +414,9 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
>                  */
>                 put_user_ex(*((u64 *)&rt_retcode), (u64 *)frame->retcode);
>         } put_user_catch(err);
> +
> +       if (current->sas_ss_flags & SS_AUTODISARM)
> +               sas_ss_reset(current);
> 
>         err |= copy_siginfo_to_user(&frame->info, &ksig->info);
>         err |= setup_sigcontext(&frame->uc.uc_mcontext, fpstate,
> diff --git a/include/linux/compat.h b/include/linux/compat.h
> index a320495fd577..f5e36931e029 100644
> --- a/include/linux/compat.h
> +++ b/include/linux/compat.h
> @@ -520,8 +520,6 @@ int __compat_save_altstack(compat_stack_t __user
> *, unsigned long);
>         put_user_ex(ptr_to_compat((void __user *)t->sas_ss_sp),
> &__uss->ss_sp); \
>         put_user_ex(t->sas_ss_flags, &__uss->ss_flags); \
>         put_user_ex(t->sas_ss_size, &__uss->ss_size); \
> -       if (t->sas_ss_flags & SS_AUTODISARM) \
> -               sas_ss_reset(t); \
>  } while (0);
> 
>  /*
> diff --git a/include/linux/signal.h b/include/linux/signal.h
> index 67ceb6d7c869..9056239787f7 100644
> --- a/include/linux/signal.h
> +++ b/include/linux/signal.h
> @@ -435,8 +435,6 @@ int __save_altstack(stack_t __user *, unsigned long);
>         put_user_ex((void __user *)t->sas_ss_sp, &__uss->ss_sp); \
>         put_user_ex(t->sas_ss_flags, &__uss->ss_flags); \
>         put_user_ex(t->sas_ss_size, &__uss->ss_size); \
> -       if (t->sas_ss_flags & SS_AUTODISARM) \
> -               sas_ss_reset(t); \
>  } while (0);
> 
>  #ifdef CONFIG_PROC_FS

Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh