Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

Peter Zijlstra
2020-02-19T16:05:04Z

On Wed, Feb 19, 2020 at 04:57:15PM +0100, Peter Zijlstra wrote:
> Something like so, I suppose...
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 6ef00eb6fbb9..543de932dc7c 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -350,14 +350,20 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code, unsign
> regs->ip == (unsigned long)native_irq_return_iret)
> {
> struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
> + unsigned long *dst = &gpregs->ip;
> + unsigned long *src = (void *)regs->dp;
> + int i, count = 5;
>
> /*
> * regs->sp points to the failing IRET frame on the
> * ESPFIX64 stack. Copy it to the entry stack. This fills
> * in gpregs->ss through gpregs->ip.
> - *
> */
> - memmove(&gpregs->ip, (void *)regs->sp, 5*8);
> + for (i = 0; i < count; i++) {
> + int idx = (dst <= src) ? i : count - i;

That's an off-by-one for going backward; 'count - 1 - i' should work
better, or I should just stop typing for today ;-)

> + dst[idx] = src[idx];
> + }
> +
> gpregs->orig_ax = 0; /* Missing (lost) #GP error code */
>
> /*