Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC_DIST

[Ingo Molnar <mingo@kernel.org>]

* Stephane Eranian <eranian@google.com> wrote:

> On Sun, Oct 21, 2012 at 6:55 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andi Kleen <ak@linux.intel.com> wrote:
> >
> >> > > > This isn't limited to admin, right? So the above turns into a DoS on the
> >> > > > console.
> >> > > >
> >> > > Ok, so how about a WARN_ON_ONCE() instead?
> >> >
> >> > That should be fine I guess ;-)
> >>
> >> imho there is need for a generic mechanism to return an error
> >> string to the user program without such hacks.
> >
> > Agreed - we could return an 'extended errno' long error return
> > value, which in reality is a pointer to an error string (in
> > perf_attr::error_str), and copy that string to user-space at
> > perf syscall return time.
> >
> I assume by perf_attr:error_str, you actually mean:
> 
> struct perf_event_attr {
>    char error_str[PERF_ERR_LEN];
> };
> 
> Right?

I don't think we should allocate space in the attr, instead we 
should use something like:

	u8 __user	*err_str;
	u32		 err_str_len;

which would be filled in by tooling with a string and a max_len 
value, and strncpy_to_user() could do the rest on the kernel 
side. [ A minor complication is that we don't have a 
strncpy_to_user() method at the moment. ]

Static strings could be handled this way.

[ Dynamic strings could be supported too with a few tricks, 
  although I doubt it matters in practice. ]

> > Thus error-string aware tooling could print the error string.
> >
> > So PMU drivers could do something obvious like:
> >
> >         return (long)"perf: INST_RETIRED.PREC_DIST only works in exclusive mode";
> >
> > The perf syscall notices these pointers by noticing that the
> > error code returned is outside the errno range.
>
> Is that always the case on all archs?

I think yes - and if not then it can be solved via some trivial 
offset value added to it on such an architecture, without 
complicating the code on normal architectures.

Thanks,

	Ingo