On Mon, May 14, 2018 at 05:02:13PM +0200, Peter Zijlstra wrote:
> On Mon, May 14, 2018 at 01:28:15PM +0200, Peter Zijlstra wrote:
> > On Mon, May 14, 2018 at 12:05:33PM +0100, Mark Rutland wrote:
>
> > > > Also note that in perf_output_put_handle(), where we write ->data_head,
> > > > the store is from an 'unsigned long'. So on 32bit that will result in a
> > > > zero high word. Similarly, in __perf_output_begin() we read ->data_tail
> > > > into an unsigned long, which will discard the high word.
> > >
> > > Ah, that's a fair point. So it's just compat userspace that this is
> > > potentially borked for. ;)
> >
> > Right.. #$$#@ compat. Hurmph.. not sure how to go about fixing that
> > there.
>
> How's this?
>
> ---
> include/linux/perf_event.h | 12 ++++++++++++
> kernel/events/core.c | 31 +++++++++++++++++++++++++++++--
> kernel/events/ring_buffer.c | 39 ++++++++++++++++++++++++++++++++++-----
> 3 files changed, 75 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index e71e99eb9a4e..7834dfb6a83b 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -517,6 +517,7 @@ typedef void (*perf_overflow_handler_t)(struct perf_event
> *,
> */
> #define PERF_EV_CAP_SOFTWARE BIT(0)
> #define PERF_EV_CAP_READ_ACTIVE_PKG BIT(1)
> +#define PERF_EV_CAP_COMPAT BIT(2)
>
> #define SWEVENT_HLIST_BITS 8
> #define SWEVENT_HLIST_SIZE (1 << SWEVENT_HLIST_BITS)
> @@ -1220,6 +1221,11 @@ static inline bool is_write_backward(struct perf_event
> *event)
> return !!event->attr.write_backward;
> }
>
> +static inline bool is_compat_event(struct perf_event *event)
> +{
> + return event->event_caps & PERF_EV_CAP_COMPAT;
> +}
> @@ -10499,6 +10523,9 @@ SYSCALL_DEFINE5(perf_event_open,
> goto err_cred;
> }
>
> + if (in_compat_syscall())
> + event->event_caps |= PERF_EV_CAP_COMPAT;
> +
After a native perf_event_open, you could pass the fd (or exec) to
another task that was compat (or vice-versa), so this wouldn't work in
that case (crazy as it may be).
I don't have a better suggestion at present, though.
Thanks,
Mark.
