Messages in this thread | | | Date | Wed, 24 Oct 2012 10:15:19 +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 8:03 PM, Ingo Molnar <mingo@kernel.org> wrote: > > > > * 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. ] > > > > Ok, but this still limits returning error string to the > perf_event_open() syscall, not read(), ioctl() and such.
Yes - but this should be enough to handle most of the cases in practice - because the richness of the various perf components is mostly exposed via the perf syscall. By the time we get to read() and ioctl() we are in a pretty well defined domain.
Also, I don't think people want the (small but nonzero) overhead of extended error reporting for read or ioctl.
> I am fine with this change. However, I think it should be > added separately from my inst_retired:prec_dist patch. It has > a broader impact.
Most definitely.
Thanks,
Ingo
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