On Tue, 27 Oct 2020 20:41:26 +0100
Peter Zijlstra <[email protected]> wrote:
> On Tue, Oct 27, 2020 at 10:15:05AM +0100, Peter Zijlstra wrote:
>
> > @@ -873,6 +866,20 @@ static __always_inline void exc_debug_ke
> > */
> > WARN_ON_ONCE(user_mode(regs));
> >
> > + if (test_thread_flag(TIF_BLOCKSTEP)) {
> > + /*
> > + * The SDM says "The processor clears the BTF flag when it
> > + * generates a debug exception." but PTRACE_BLOCKSTEP requested
> > + * it for userspace, but we just took a kernel #DB, so re-set
> > + * BTF.
> > + */
> > + unsigned long debugctl;
> > +
> > + rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
> > + debugctl |= DEBUGCTLMSR_BTF;
> > + wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
> > + }
> > +
> > /*
> > * Catch SYSENTER with TF set and clear DR_STEP. If this hit a
> > * watchpoint at the same time then that will still be handled.
>
> Masami, how does BTF interact with !optimized kprobes that single-step?
Good question, BTF is cleared right before single-stepping and restored
after single-stepping. It will be done accoding to TIF_BLOCKSTEP bit as below.
(in arch/x86/kernel/kprobes/core.c)
static nokprobe_inline void clear_btf(void)
{
if (test_thread_flag(TIF_BLOCKSTEP)) {
unsigned long debugctl = get_debugctlmsr();
debugctl &= ~DEBUGCTLMSR_BTF;
update_debugctlmsr(debugctl);
}
}
static nokprobe_inline void restore_btf(void)
{
if (test_thread_flag(TIF_BLOCKSTEP)) {
unsigned long debugctl = get_debugctlmsr();
debugctl |= DEBUGCTLMSR_BTF;
update_debugctlmsr(debugctl);
}
}
Hrm, so it seems that we do same ... maybe we don't need clear_btf() too?
>
> The best answer I can come up with is 'poorly' :/
Is this what you expected? :)
Thank you,
--
Masami Hiramatsu <[email protected]>
