* Stephane Eranian <eran...@google.com> wrote:
> On Sun, Oct 21, 2012 at 6:55 PM, Ingo Molnar <mi...@kernel.org> wrote:
> >
> > * Andi Kleen <a...@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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