[PATCH 2/2] RISC-V: Add some comments about the shadow and overflow stacks

[Jisheng Zhang]

On Tue, Nov 29, 2022 at 06:35:15PM -0800, Palmer Dabbelt wrote:
> It took me a while to page all this back in when trying to review the
> recent spin_shadow_stack, so I figured I'd just write up some comments.
> 
> Signed-off-by: Palmer Dabbelt <palmer at rivosinc.com>

Reviewed-by: Jisheng Zhang <jszhang at kernel.org>

> ---
>  arch/riscv/kernel/traps.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index acdfcacd7e57..336d4aadadb1 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -200,18 +200,18 @@ void __init trap_init(void)
>  }
>  
>  #ifdef CONFIG_VMAP_STACK
> +/*
> + * Extra stack space that allows us to provide panic messages when the kernel
> + * has overflowed its stack.
> + */
>  static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
>  		overflow_stack)__aligned(16);
>  /*
> - * shadow stack, handled_ kernel_ stack_ overflow(in kernel/entry.S) is used
> - * to get per-cpu overflow stack(get_overflow_stack).
> + * A temporary stack for use by handle_kernel_stack_overflow.  This is used so
> + * we can call into C code to get the per-hart overflow stack.  Usage of this
> + * stack must be protected by spin_shadow_stack.
>   */
>  long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
> -asmlinkage unsigned long get_overflow_stack(void)
> -{
> -	return (unsigned long)this_cpu_ptr(overflow_stack) +
> -		OVERFLOW_STACK_SIZE;
> -}
>  
>  /*
>   * A pseudo spinlock to protect the shadow stack from being used by multiple
> @@ -222,6 +222,12 @@ asmlinkage unsigned long get_overflow_stack(void)
>   */
>  unsigned long spin_shadow_stack;
>  
> +asmlinkage unsigned long get_overflow_stack(void)
> +{
> +	return (unsigned long)this_cpu_ptr(overflow_stack) +
> +		OVERFLOW_STACK_SIZE;
> +}
> +
>  asmlinkage void handle_bad_stack(struct pt_regs *regs)
>  {
>  	unsigned long tsk_stk = (unsigned long)current->stack;
> -- 
> 2.38.1
>