Re: [PATCH V5 08/11] riscv: Support HAVE_IRQ_EXIT_ON_IRQ_STACK

Peter Zijlstra
2022-09-19T13:45:34Z

On Sun, Sep 18, 2022 at 11:52:43AM -0400, guoren@kernel.org wrote:

> +ENTRY(call_on_stack)
> + /* Create a frame record to save our ra and fp */
> + addi sp, sp, -RISCV_SZPTR
> + REG_S ra, (sp)
> + addi sp, sp, -RISCV_SZPTR
> + REG_S fp, (sp)
> +
> + /* Save sp in fp */
> + move fp, sp
> +
> + /* Move to the new stack and call the function there */
> + li a3, IRQ_STACK_SIZE
> + add sp, a1, a3
> + jalr a2
> +
> + /*
> + * Restore sp from prev fp, and fp, ra from the frame
> + */
> + move sp, fp
> + REG_L fp, (sp)
> + addi sp, sp, RISCV_SZPTR
> + REG_L ra, (sp)
> + addi sp, sp, RISCV_SZPTR
> + ret
> +ENDPROC(call_on_stack)

IIRC x86_64 moved away from a stack-switch function like this because it
presents a convenient exploit gadget.

I'm not much of an exploit writer and I've no idea how effective our
inline stategy is, perhaps other can comment.