Re: [PATCH v2 2/4] arm64, scs: save scs_sp values per-cpu when switching stacks

Mark Rutland
2022-03-31T09:24:39Z

On Wed, Mar 23, 2022 at 04:32:53PM +0100, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
>
> When an interrupt happens, the current Shadow Call Stack (SCS) pointer
> is switched to a per-interrupt one stored in a per-CPU variable. The old
> pointer is then saved on the normal stack and restored when the interrupt
> is handled.
>
> To collect the current stack trace based on SCS when the interrupt is
> being handled, we need to know the SCS pointers that belonged to the
> task and potentially other interrupts that were interrupted.
>
> Instead of trying to retrieve the SCS pointers from the stack, change
> interrupt handlers (for hard IRQ, Normal and Critical SDEI) to save the
> previous SCS pointer in a per-CPU variable.

I'm *really* not keen on *always* poking this in the entry code for the
uncommon case of unwind. It complicates the entry code and means we're always
paying a cost for potentially no benefit. At a high-level, I don't think this
is the right approach.

For the regular unwinder, I want to rework things such that we can identify
exception boundaries and look into the regs (e.g. so that we can recover the
PC+LR+FP and avoid duplicating part of this in a frame record), and I'd much
prefer that we did the same here.

Thanks,
Mark.

> Note that interrupts stack. A task can be interrupted by a hard IRQ,
> which then can interrupted by a normal SDEI, etc. This is handled by
> using a separate per-CPU variable for each interrupt type.
>
> Also reset the saved SCS pointer when exiting the interrupt. This allows
> checking whether we should include any interrupt frames when collecting
> the stack trace. While we could use in_hardirq(), there seems to be no
> easy way to check whether we are in an SDEI handler. Directly checking
> the per-CPU variables for being non-zero is more resilient.
>
> Also expose both the the added saved SCS variables and the existing SCS
> base variables in arch/arm64/include/asm/scs.h so that the stack trace
> collection impementation can use them.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
> arch/arm64/include/asm/assembler.h | 12 ++++++++++++
> arch/arm64/include/asm/scs.h | 13 ++++++++++++-
> arch/arm64/kernel/entry.S | 28 ++++++++++++++++++++++++----
> arch/arm64/kernel/irq.c | 4 +---
> arch/arm64/kernel/sdei.c | 5 ++---
> 5 files changed, 51 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index 8c5a61aeaf8e..ca018e981d13 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -270,6 +270,18 @@ alternative_endif
> ldr \dst, [\dst, \tmp]
> .endm
>
> + /*
> + * @src: Register whose value gets stored in sym
> + * @sym: The name of the per-cpu variable
> + * @tmp0: Scratch register
> + * @tmp1: Another scratch register
> + */
> + .macro str_this_cpu src, sym, tmp0, tmp1
> + adr_l \tmp0, \sym
> + get_this_cpu_offset \tmp1
> + str \src, [\tmp0, \tmp1]
> + .endm
> +
> /*
> * vma_vm_mm - get mm pointer from vma pointer (vma->vm_mm)
> */
> diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
> index 8297bccf0784..2bb2b32f787b 100644
> --- a/arch/arm64/include/asm/scs.h
> +++ b/arch/arm64/include/asm/scs.h
> @@ -24,6 +24,17 @@
> .endm
> #endif /* CONFIG_SHADOW_CALL_STACK */
>
> -#endif /* __ASSEMBLY __ */
> +#else /* __ASSEMBLY__ */
> +
> +#include <linux/percpu.h>
> +
> +DECLARE_PER_CPU(unsigned long *, irq_shadow_call_stack_ptr);
> +DECLARE_PER_CPU(unsigned long *, irq_shadow_call_stack_saved_ptr);
> +DECLARE_PER_CPU(unsigned long *, sdei_shadow_call_stack_normal_ptr);
> +DECLARE_PER_CPU(unsigned long *, sdei_shadow_call_stack_normal_saved_ptr);
> +DECLARE_PER_CPU(unsigned long *, sdei_shadow_call_stack_critical_ptr);
> +DECLARE_PER_CPU(unsigned long *, sdei_shadow_call_stack_critical_saved_ptr);
> +
> +#endif /* __ASSEMBLY__ */
>
> #endif /* _ASM_SCS_H */
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index ede028dee81b..1c62fecda172 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -880,7 +880,8 @@ NOKPROBE(ret_from_fork)
> */
> SYM_FUNC_START(call_on_irq_stack)
> #ifdef CONFIG_SHADOW_CALL_STACK
> - stp scs_sp, xzr, [sp, #-16]!
> + /* Save the current SCS pointer and load the per-IRQ one. */
> + str_this_cpu scs_sp, irq_shadow_call_stack_saved_ptr, x15, x17
> ldr_this_cpu scs_sp, irq_shadow_call_stack_ptr, x17
> #endif
> /* Create a frame record to save our LR and SP (implicit in FP) */
> @@ -902,7 +903,9 @@ SYM_FUNC_START(call_on_irq_stack)
> mov sp, x29
> ldp x29, x30, [sp], #16
> #ifdef CONFIG_SHADOW_CALL_STACK
> - ldp scs_sp, xzr, [sp], #16
> + /* Restore saved SCS pointer and reset the saved value. */
> + ldr_this_cpu scs_sp, irq_shadow_call_stack_saved_ptr, x17
> + str_this_cpu xzr, irq_shadow_call_stack_saved_ptr, x15, x17
> #endif
> ret
> SYM_FUNC_END(call_on_irq_stack)
> @@ -1024,11 +1027,16 @@ SYM_CODE_START(__sdei_asm_handler)
> #endif
>
> #ifdef CONFIG_SHADOW_CALL_STACK
> - /* Use a separate shadow call stack for normal and critical events */
> + /*
> + * Use a separate shadow call stack for normal and critical events.
> + * Save the current SCS pointer and load the per-SDEI one.
> + */
> cbnz w4, 3f
> + str_this_cpu src=scs_sp, sym=sdei_shadow_call_stack_normal_saved_ptr, tmp0=x5, tmp1=x6
> ldr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_normal_ptr, tmp=x6
> b 4f
> -3: ldr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_critical_ptr, tmp=x6
> +3: str_this_cpu src=scs_sp, sym=sdei_shadow_call_stack_critical_saved_ptr, tmp0=x5, tmp1=x6
> + ldr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_critical_ptr, tmp=x6
> 4:
> #endif
>
> @@ -1062,6 +1070,18 @@ SYM_CODE_START(__sdei_asm_handler)
> ldp lr, x1, [x4, #SDEI_EVENT_INTREGS + S_LR]
> mov sp, x1
>
> +#ifdef CONFIG_SHADOW_CALL_STACK
> + /* Restore saved SCS pointer and reset the saved value. */
> + ldrb w5, [x4, #SDEI_EVENT_PRIORITY]
> + cbnz w5, 5f
> + ldr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_normal_saved_ptr, tmp=x6
> + str_this_cpu src=xzr, sym=sdei_shadow_call_stack_normal_saved_ptr, tmp0=x5, tmp1=x6
> + b 6f
> +5: ldr_this_cpu dst=scs_sp, sym=sdei_shadow_call_stack_critical_saved_ptr, tmp=x6
> + str_this_cpu src=xzr, sym=sdei_shadow_call_stack_critical_saved_ptr, tmp0=x5, tmp1=x6
> +6:
> +#endif
> +
> mov x1, x0 // address to complete_and_resume
> /* x0 = (x0 <= SDEI_EV_FAILED) ?
> * EVENT_COMPLETE:EVENT_COMPLETE_AND_RESUME
> diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
> index bda49430c9ea..4199f900714a 100644
> --- a/arch/arm64/kernel/irq.c
> +++ b/arch/arm64/kernel/irq.c
> @@ -28,11 +28,9 @@ DEFINE_PER_CPU(struct nmi_ctx, nmi_contexts);
>
> DEFINE_PER_CPU(unsigned long *, irq_stack_ptr);
>
> -
> -DECLARE_PER_CPU(unsigned long *, irq_shadow_call_stack_ptr);
> -
> #ifdef CONFIG_SHADOW_CALL_STACK
> DEFINE_PER_CPU(unsigned long *, irq_shadow_call_stack_ptr);
> +DEFINE_PER_CPU(unsigned long *, irq_shadow_call_stack_saved_ptr);
> #endif
>
> static void init_irq_scs(void)
> diff --git a/arch/arm64/kernel/sdei.c b/arch/arm64/kernel/sdei.c
> index d20620a1c51a..269adcb9e854 100644
> --- a/arch/arm64/kernel/sdei.c
> +++ b/arch/arm64/kernel/sdei.c
> @@ -39,12 +39,11 @@ DEFINE_PER_CPU(unsigned long *, sdei_stack_normal_ptr);
> DEFINE_PER_CPU(unsigned long *, sdei_stack_critical_ptr);
> #endif
>
> -DECLARE_PER_CPU(unsigned long *, sdei_shadow_call_stack_normal_ptr);
> -DECLARE_PER_CPU(unsigned long *, sdei_shadow_call_stack_critical_ptr);
> -
> #ifdef CONFIG_SHADOW_CALL_STACK
> DEFINE_PER_CPU(unsigned long *, sdei_shadow_call_stack_normal_ptr);
> +DEFINE_PER_CPU(unsigned long *, sdei_shadow_call_stack_normal_saved_ptr);
> DEFINE_PER_CPU(unsigned long *, sdei_shadow_call_stack_critical_ptr);
> +DEFINE_PER_CPU(unsigned long *, sdei_shadow_call_stack_critical_saved_ptr);
> #endif
>
> static void _free_sdei_stack(unsigned long * __percpu *ptr, int cpu)
> --
> 2.25.1
>