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SubjectRe: Fixing perf top --user shortcoming was: Re: [GIT PULL 0/9] perf/core improvements and fixes

* Arnaldo Carvalho de Melo <acme@infradead.org> wrote:

> > > ptrace_may_access(task, PTRACE_MODE_READ) fails for some
> > > tasks owned by the user because, IIRC, in
> > > __ptrace_may_access:
>
> > Which tasks are these, are they privileged in any sense?
>
> IIRC one of them was a child of sshd, that runs as root and
> then changes the child ownership to the user logging in.

It's probably privileged then - or at least not sufficiently
deprivileged.

Skipping them ought to be the right solution - it's not like
such tasks tend to create a lot of overhead worth profiling.
They are also not debuggable via gdb so they are not part of the
user's development session and such.

> I'll continue investigation but probably for now the first
> thing to do is to just remove them from the thread_map when
> they return -EPERM.

Yeah. Maybe warn about them in verbose mode or such.

> > If yes and if most of the 'real' tasks a user have can be
> > profiled just fine then i think we should just skip the
> > privileged tasks and not abort the profiling session?
>
> Yeah, that can be done, while debugging I'll emit a warning
> with the resulting thread_map of "special tasks" to figure out
> what makes them special.

Ok, sounds great!

Thanks,

Ingo


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