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

[Masami Hiramatsu]

On Wed, 26 May 2021 10:23:18 +0530
Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx> wrote:

> 
> 
> 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.

Ah, yes. In that case, perf probe must not use the base symbol.
(like -D option)
OK, then, let's merge this fix.

Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thank you,

> 
> 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


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>