Re: [PATCH 1/2] powerpc/irq: don't use current_stack_pointer() in check_stack_overflow()

[Michael Ellerman <mpe@ellerman.id.au>]

Christophe Leroy <christophe.leroy@c-s.fr> writes:

> current_stack_pointer() doesn't return the stack pointer, but the
> caller's stack frame. See commit bfe9a2cfe91a ("powerpc: Reimplement
> __get_SP() as a function not a define") and commit acf620ecf56c
> ("powerpc: Rename __get_SP() to current_stack_pointer()") for details.
>
> The purpose of check_stack_overflow() is to verify that the stack has
> not overflowed.
>
> To really know whether the stack pointer is still within boundaries,
> the check must be done directly on the value of r1.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> ---
>  arch/powerpc/kernel/irq.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> index bb34005ff9d2..4d468d835558 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -599,9 +599,8 @@ u64 arch_irq_stat_cpu(unsigned int cpu)
>  static inline void check_stack_overflow(void)
>  {
>  #ifdef CONFIG_DEBUG_STACKOVERFLOW
> -	long sp;
> -
> -	sp = current_stack_pointer() & (THREAD_SIZE-1);
> +	register unsigned long r1 asm("r1");
> +	long sp = r1 & (THREAD_SIZE - 1);

This appears to work but seems to be "unsupported" by GCC, and clang
actually complains about it:

  /linux/arch/powerpc/kernel/irq.c:603:12: error: variable 'r1' is uninitialized when used here [-Werror,-Wuninitialized]
          long sp = r1 & (THREAD_SIZE - 1);
                    ^~

The GCC docs say:

  The only supported use for this feature is to specify registers for
  input and output operands when calling Extended asm (see Extended
  Asm).

https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Local-Register-Variables.html#Local-Register-Variables


If I do this it seems to work, but feels a little dicey:

	asm ("" : "=r" (r1));
	sp = r1 & (THREAD_SIZE - 1);


Generated code looks OK ish:

clang:

        sp = r1 & (THREAD_SIZE - 1);
    22e0:       a0 04 24 78     clrldi  r4,r1,50
        if (unlikely(sp < 2048)) {
    22e4:       ff 07 24 28     cmpldi  r4,2047
    22e8:       58 00 81 40     ble     2340 <do_IRQ+0xe0>


gcc:
	if (unlikely(sp < 2048)) {
    2eb4:	00 38 28 70 	andi.   r8,r1,14336
...
    2ecc:	24 00 82 40 	bne     c000000000002ef0 <do_IRQ+0xa0>


cheers