[PATCH 20/30] ARM: kernel: use PC relative symbol references in suspend/resume code

[Nicolas Pitre]

On Mon, 14 Aug 2017, Ard Biesheuvel wrote:

> On 14 August 2017 at 17:02, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> > On Mon, 14 Aug 2017, Ard Biesheuvel wrote:
> >
> >> Replace some unnecessary absolute references with relative ones. Also,
> >> to prepare for runtime relocation, which occurs with the caches on,
> >> defer taking the absolute address of cpu_resume_after_mmu() until after
> >> the MMU is enabled.
> >>
> >> Cc: Russell King <linux at armlinux.org.uk>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> >> ---
> >>  arch/arm/kernel/sleep.S | 11 +++++------
> >>  1 file changed, 5 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/arch/arm/kernel/sleep.S b/arch/arm/kernel/sleep.S
> >> index 3026b119d3ff..9efd1c7d3552 100644
> >> --- a/arch/arm/kernel/sleep.S
> >> +++ b/arch/arm/kernel/sleep.S
> >> @@ -60,18 +60,17 @@
> >>  ENTRY(__cpu_suspend)
> >>       stmfd   sp!, {r4 - r11, lr}
> >>  #ifdef MULTI_CPU
> >> -     ldr     r10, =processor
> >> -     ldr     r4, [r10, #CPU_SLEEP_SIZE] @ size of CPU sleep state
> >> +     ldr_l   r4, processor + CPU_SLEEP_SIZE  @ size of CPU sleep state
> >>  #else
> >> -     ldr     r4, =cpu_suspend_size
> >> +     adr_l   r4, cpu_suspend_size
> >>  #endif
> >>       mov     r5, sp                  @ current virtual SP
> >>       add     r4, r4, #12             @ Space for pgd, virt sp, phys resume fn
> >>       sub     sp, sp, r4              @ allocate CPU state on stack
> >> -     ldr     r3, =sleep_save_sp
> >> +     adr_l   r3, sleep_save_sp
> >>       stmfd   sp!, {r0, r1}           @ save suspend func arg and pointer
> >>       ldr     r3, [r3, #SLEEP_SAVE_SP_VIRT]
> >> -     ALT_SMP(ldr r0, =mpidr_hash)
> >> +     ALT_SMP(adr_l r0, mpidr_hash)
> >>       ALT_UP_B(1f)
> >
> > The above is dangerous. adr_l expands to more than one instruction which
> > is not what ALT_SMP() was designed for. Here it might happen to work
> > anyway because it is combined with ALT_UP_B() but with ALT_UP() it
> > wouldn't. This is a mistake waiting to happen.
> >
> 
> OK. I will use the opencoded sequence instead in this case. I.e.,
> 
> -       ALT_SMP(ldr r0, =mpidr_hash)
> +0:     ALT_SMP(adr r0, 2f)
>         ALT_UP_B(1f)
> +       ldr     r1, [r0]
> +       add     r0, r0, r1
> 
> and
> 
>  ENDPROC(__cpu_suspend)
> +       .align  2
> +2:     .long   mpidr_hash - .
>         .ltorg
> 

Yeah... I see no way around it.

And if you make this particular case into a commit of its own, then the 
commit log may carry the above reasoning.


Nicolas