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

Guo Ren
2022-12-01T13:17:44Z

On Wed, Nov 30, 2022 at 10:35 AM Palmer Dabbelt <palmer@rivosinc.com> 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@rivosinc.com>
> ---
> 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.
Reviewed-by: Guo Ren <guoren@kernel.org>

> */
> 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
>

--
Best Regards
Guo Ren