On Sun, Oct 21, 2012 at 8:03 PM, Ingo Molnar <[email protected]> wrote: > > * Stephane Eranian <[email protected]> wrote: > >> On Sun, Oct 21, 2012 at 6:55 PM, Ingo Molnar <[email protected]> wrote: >> > >> > * Andi Kleen <[email protected]> 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.
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. >> > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
