Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code

Peter Zijlstra
2020-02-26T08:17:40Z

On Tue, Feb 25, 2020 at 09:43:46PM -0800, Andy Lutomirski wrote:
> On 2/25/20 2:08 PM, Thomas Gleixner wrote:
> > Now that the C entry points are safe, move the irq flags tracing code into
> > the entry helper.
>
> I'm so confused.
>
> >
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > ---
> > arch/x86/entry/common.c | 5 +++++
> > arch/x86/entry/entry_32.S | 12 ------------
> > arch/x86/entry/entry_64.S | 2 --
> > arch/x86/entry/entry_64_compat.S | 18 ------------------
> > 4 files changed, 5 insertions(+), 32 deletions(-)
> >
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -57,6 +57,11 @@ static inline void enter_from_user_mode(
> > */
> > static __always_inline void syscall_entry_fixups(void)
> > {
> > + /*
> > + * Usermode is traced as interrupts enabled, but the syscall entry
> > + * mechanisms disable interrupts. Tell the tracer.
> > + */
> > + trace_hardirqs_off();
>
> Your earlier patches suggest quite strongly that tracing isn't safe
> until enter_from_user_mode(). But trace_hardirqs_off() calls
> trace_irq_disable_rcuidle(), which looks [0] like a tracepoint.
>
> Did you perhaps mean to do this *after* enter_from_user_mode()?

aside from the fact that enter_from_user_mode() itself also has a
tracepoint, the crucial detail is that we must not trace/kprobe the
function calling this.

Specifically for #PF, because we need read_cr2() before this. See later
patches.