Messages in this thread | | | Date | Sun, 21 Oct 2012 18:55:23 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC_DIST |
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* 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.
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.
Old userspace will get a -EINVAL and no string copied into the error string buffer.
New userspace would get the error string copied into perf_attr::error_str, plus a 'normal' -EINVAL error code.
The only cost on the kernel side is to make sure all "string errors" are returned as long.
Thanks,
Ingo
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