Re: [PATCH] perf probe: Provide more detail with relocation warning

[Ravi Bangoria]

On 5/25/21 6:18 PM, Masami Hiramatsu wrote:
> On Tue, 25 May 2021 10:07:44 +0530
> Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx> wrote:
>
> > When run as normal user with default sysctl kernel.kptr_restrict=0
> > and kernel.perf_event_paranoid=2, perf probe fails with:
> >
> >    $ ./perf probe move_page_tables
> >    Relocated base symbol is not found!
> >
> > The warning message is not much informative. The reason perf fails
> > is because /proc/kallsyms is restricted by perf_event_paranoid=2
> > for normal user and thus perf fails to read relocated address of
> > the base symbol.
> >
> > Tweaking kptr_restrict and perf_event_paranoid can change the
> > behavior of perf probe. Also, running as root or privileged user
> > works too. Add these details in the warning message.
> >
> > Plus, kmap->ref_reloc_sym might not be always set even if
> > host_machine is initialized. Above is the example of the same.
> > Remove that comment.
>
> Yes, those are restricted in some cases. Anyway without priviledged
> (super) user, perf probe can not set the probe in ftrace.
>
> Hmm, I think it should check the effective user-id at first. If it
> is not super user and the action will access tracefs and kallsyms,
> it should warn at that point.

If kptr_restrict=2, perf probe fails with same error even for root user.
That's why I thought to just change this warning message.

Different combinations of privilege, perf_event_paranoid, kptr_restrict:

  Normal/Root user
   |   perf_event_paranoid
   V    V   kptr_restrict        perf probe error
  ----------------------------------------------------------------
   N   -1    0     Failed to open kprobe_events: Permission denied
   N    0    0     Failed to open kprobe_events: Permission denied
   N    1    0     Failed to open kprobe_events: Permission denied
   N    2    0     Relocated base symbol is not found!
  
   N   -1    1     Relocated base symbol is not found!
   N    0    1     Relocated base symbol is not found!
   N    1    1     Relocated base symbol is not found!
   N    2    1     Relocated base symbol is not found!
  
   N   -1    2     Relocated base symbol is not found!
   N    0    2     Relocated base symbol is not found!
   N    1    2     Relocated base symbol is not found!
   N    2    2     Relocated base symbol is not found!
  
   R   -1    0     No error.
   R    0    0     No error.
   R    1    0     No error.
   R    2    0     No error.
  
   R   -1    1     No error.
   R    0    1     No error.
   R    1    1     No error.
   R    2    1     No error.
  
   R   -1    2     Relocated base symbol is not found!
   R    0    2     Relocated base symbol is not found!
   R    1    2     Relocated base symbol is not found!
   R    2    2     Relocated base symbol is not found!

Ravi