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SubjectRe: objtool warning "uses BP as a scratch register" with clang-9
On Wed, Aug 28, 2019 at 05:13:59PM +0200, Arnd Bergmann wrote:
> 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?

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.

--
Josh

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