Subject: [PATCH 2/2] arm64: use memset to clear BSS

From: [email-masked] (Mark Rutland)
Date: Wed, 6 Jan 2016 11:40:39 +0000

On Wed, Jan 06, 2016 at 12:12:45PM +0100, Ard Biesheuvel wrote:
> On 6 January 2016 at 12:05, Mark Rutland <[email-masked]> wrote:
> > Currently we use an open-coded memzero to clear the BSS. As it is a
> > trivial implementation, it is sub-optimal.
> >
> > Our optimised memset doesn't use the stack, is position-independent, and
> > for the memzero case can use of DC ZVA to clear large blocks
> > efficiently. In __mmap_switched the MMU is on and there are no live
> > caller-saved registers, so we can safely call an uninstrumented memset.
> >
> > This patch changes __mmap_switched to use memset when clearing the BSS.
> > We use the __pi_memset alias so as to avoid any instrumentation in all
> > kernel configurations. As with the head symbols, we must get the linker
> > to generate __bss_size, as there is no ELF relocation for the
> > subtraction of two symbols.
> >
> > Signed-off-by: Mark Rutland <[email-masked]>
> > Cc: Ard Biesheuvel <[email-masked]>
> > Cc: Catalin Marinas <[email-masked]>
> > Cc: Marc Zyngier <[email-masked]>
> > Cc: Will Deacon <[email-masked]>
> > ---
> >  arch/arm64/kernel/head.S  | 14 ++++++--------
> >  arch/arm64/kernel/image.h |  2 ++
> >  2 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index 23cfc08..247a97b 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -415,14 +415,12 @@ ENDPROC(__create_page_tables)
> >   */
> >         .set    initial_sp, init_thread_union + THREAD_START_SP
> >  __mmap_switched:
> > -       adr_l   x6, __bss_start
> > -       adr_l   x7, __bss_stop
> > -
> > -1:     cmp     x6, x7
> > -       b.hs    2f
> > -       str     xzr, [x6], #8                   // Clear BSS
> > -       b       1b
> > -2:
> > +       // clear BSS
> > +       adr_l   x0, __bss_start
> > +       mov     x1, xzr
> > +       mov_l   x2, __bss_size
> 
> Is it such a big deal to do
> 
> adr_l x2, __bss_stop
> sub x2, x2, x0
> 
> instead?

I'm happy either way.

It no-one else has a use for mov_l I'll drop it and move to that.

> Either way:
> Reviewed-by: Ard Biesheuvel <[email-masked]>

Thanks!

Mark.