Messages in this thread | | | Date | Sun, 21 Oct 2012 20:03:41 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC_DIST |
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* 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
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