On Wed, 28 Oct 2020 10:59:19 +0100 Peter Zijlstra <[email protected]> wrote:
> On Wed, Oct 28, 2020 at 06:20:25PM +0900, Masami Hiramatsu wrote: > > 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? > > No, I think you do very much need clear_btf(). But with my patch perhaps > restore_btf() is no longer needed. Is there only a single single-step > between setup_singlestep() and resume_execution() ? (I think so). It depends on what the single step instruction does, if it access to the non-present memory (like user-memory) it kicks the fault handler instead of debug handler. (e.g. putting a kprobe on the fixup source address in copy_from_user() ) Hmm, on this path, it seems not calling restore_btf()... Thanks, > Also, I note that we should employ get_debugctlmsr() more consistently. > > > > The best answer I can come up with is 'poorly' :/ > > > > Is this what you expected? :) > > Nah, I missed the above, you seems to do the right thing. -- Masami Hiramatsu <[email protected]>
