Messages in this thread | | | Date | Thu, 29 Aug 2019 18:24:39 -0500 | From | Josh Poimboeuf <> | Subject | Re: objtool warning "uses BP as a scratch register" with clang-9 |
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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@arndb.de> wrote: > > On Wed, Aug 28, 2019 at 5:22 PM Josh Poimboeuf <jpoimboe@redhat.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@redhat.com>
-- Josh
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