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SubjectRe: [PATCH v15 04/10] arm64: Kprobes with single stepping support
On Wed, Jul 20, 2016 at 10:36:08AM +0100, Marc Zyngier wrote:
> On 08/07/16 17:35, David Long wrote:
> > +void __kprobes jprobe_return(void)
> > +{
> > + struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> > +
> > + /*
> > + * Jprobe handler return by entering break exception,
> > + * encoded same as kprobe, but with following conditions
> > + * -a magic number in x0 to identify from rest of other kprobes.
> > + * -restore stack addr to original saved pt_regs
> > + */
> > + asm volatile ("ldr x0, [%0]\n\t"
> > + "mov sp, x0\n\t"
> > + ".globl jprobe_return_break\n\t"
> > + "jprobe_return_break:\n\t"
> > + "brk %1\n\t"
> > + :
> > + : "r"(&kcb->jprobe_saved_regs.sp),
> > + "I"(BRK64_ESR_KPROBES)
> > + : "memory");
>
> A couple of remarks here:
> - the comment seems wrong, as you load the stack pointer in X0, nothing
> else, and seem to identify the jprobe by looking at the PC, not X0.
> - using explicit registers is really ugly. How about something like this
> instead:
>
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index c89811d..823cf92 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -513,13 +513,12 @@ void __kprobes jprobe_return(void)
> * -a magic number in x0 to identify from rest of other kprobes.
> * -restore stack addr to original saved pt_regs
> */
> - asm volatile ("ldr x0, [%0]\n\t"
> - "mov sp, x0\n\t"
> + asm volatile ("mov sp, %0\n\t"
> ".globl jprobe_return_break\n\t"
> "jprobe_return_break:\n\t"
> "brk %1\n\t"
> :
> - : "r"(&kcb->jprobe_saved_regs.sp),
> + : "r" (kcb->jprobe_saved_regs.sp),
> "I"(BRK64_ESR_KPROBES)
> : "memory");
> }

The comment indeed doesn't make any sense. Is x0 useful at all?
Otherwise, Marc's fixup looks better.

> though hijacking SP in the middle of a C function still feels pretty fragile.

It may not be that bad if this function is never supposed to return.
However, I no longer hit jprobe_return() in my tests, it fails earlier
when it hits the function entry breakpoint. One difference from the
default Kprobes tests is that tcp_rcv_established() runs in interrupt
context on the IRQ stack. Maybe setjmp_pre_handler() doesn't set things
up properly.

Also, is setjmp_pre_handler() guaranteed to run in a non-preemptible
context? It uses MIN_STACK_SIZE macro which does a
raw_smp_processor_id().

--
Catalin

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