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

[Arnd Bergmann]

On Wed, Aug 28, 2019 at 5:28 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wed, Aug 28, 2019 at 5:22 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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