From: Masami Hiramatsu <mhiramat@kernel.org>
To: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: acme@kernel.org, jolsa@redhat.com, linux-kernel@vger.kernel.org,
aneesh.kumar@linux.ibm.com
Subject: Re: [PATCH] perf probe: Provide more detail with relocation warning
Date: Wed, 26 May 2021 15:33:40 +0900 [thread overview]
Message-ID: <20210526153340.a49ba8292f201493990f210c@kernel.org> (raw)
In-Reply-To: <05e32c82-1009-03ba-d973-8b1bc0582ce2@linux.ibm.com>
On Wed, 26 May 2021 10:23:18 +0530
Ravi Bangoria <ravi.bangoria@linux.ibm.com> wrote:
>
>
> On 5/25/21 6:18 PM, Masami Hiramatsu wrote:
> > On Tue, 25 May 2021 10:07:44 +0530
> > Ravi Bangoria <ravi.bangoria@linux.ibm.com> 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@kernel.org>
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@kernel.org>
next prev parent reply other threads:[~2021-05-26 6:33 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-25 4:37 [PATCH] perf probe: Provide more detail with relocation warning Ravi Bangoria
2021-05-25 12:48 ` Masami Hiramatsu
2021-05-26 4:53 ` Ravi Bangoria
2021-05-26 6:33 ` Masami Hiramatsu [this message]
2021-05-26 12:56 ` Arnaldo Carvalho de Melo
2021-05-26 14:20 ` Masami Hiramatsu
2021-06-02 11:52 ` Arnaldo Carvalho de Melo
2021-06-02 12:12 ` Masami Hiramatsu
2021-06-02 13:15 ` Arnaldo Carvalho de Melo
2021-05-26 9:01 ` [PATCH] tools/perf: doc: Add permission and sysctl notice Masami Hiramatsu
2021-05-26 9:50 ` Ravi Bangoria
2021-05-26 13:16 ` Masami Hiramatsu
2021-05-26 14:51 ` [PATCH v2] " Masami Hiramatsu
2021-06-02 11:53 ` Arnaldo Carvalho de Melo
2021-06-02 13:51 ` Ravi Bangoria
2021-06-04 13:25 ` Arnaldo Carvalho de Melo
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