Messages in this thread | | | Date | Wed, 2 Jun 2021 21:12:48 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH] perf probe: Provide more detail with relocation warning |
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On Wed, 2 Jun 2021 08:52:41 -0300 Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> Em Wed, May 26, 2021 at 11:20:20PM +0900, Masami Hiramatsu escreveu: > > On Wed, 26 May 2021 09:56:29 -0300 > > Arnaldo Carvalho de Melo <acme@kernel.org> wrote: > > > > > Em Wed, May 26, 2021 at 03:33:40PM +0900, Masami Hiramatsu escreveu: > > > > 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> > > > > > > Thanks, applied as it improves the current situation. > > > > > > But as a follow up, to further improve this, we can reuse what 'perf trace' has: > > > > > > $ perf trace sleep 1 > > > Error: No permissions to read /sys/kernel/tracing/events/raw_syscalls/sys_(enter|exit) > > > Hint: Try 'sudo mount -o remount,mode=755 /sys/kernel/tracing/' > > > $ sudo mount -o remount,mode=755 /sys/kernel/tracing/ > > > $ perf trace sleep 1 > > > Error: Permission denied. > > > Hint: Check /proc/sys/kernel/perf_event_paranoid setting. > > > Hint: For your workloads it needs to be <= 1 > > > Hint: For system wide tracing it needs to be set to -1. > > > Hint: Try: 'sudo sh -c "echo -1 > /proc/sys/kernel/perf_event_paranoid"' > > > Hint: The current value is 2. > > > $ > > > > OK, let me check this. > > BTW, does perf_event_paranoid affect only perf syscall (and kallsyms), > > not the tracefs correct? > > > > > I.e. go the extra step and show what the current value is and what it > > > needs to be to achieve what is being attempted. > > > > > > IOW combine error message with relevant documentation, to save steps. > > > > > > See what 'perf top' does for an unpriv user: > > > > > > $ perf top --stdio > > > Error: > > > Access to performance monitoring and observability operations is limited. > > > Enforced MAC policy settings (SELinux) can limit access to performance > > > monitoring and observability operations. Inspect system audit records for > > > more perf_event access control information and adjusting the policy. > > > Consider adjusting /proc/sys/kernel/perf_event_paranoid setting to open > > > access to performance monitoring and observability operations for processes > > > without CAP_PERFMON, CAP_SYS_PTRACE or CAP_SYS_ADMIN Linux capability. > > > More information can be found at 'Perf events and tool security' document: > > > https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html > > > perf_event_paranoid setting is 2: > > > -1: Allow use of (almost) all events by all users > > > Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK > > > >= 0: Disallow raw and ftrace function tracepoint access > > > >= 1: Disallow CPU event access > > > >= 2: Disallow kernel profiling > > > To make the adjusted perf_event_paranoid setting permanent preserve it > > > in /etc/sysctl.conf (e.g. kernel.perf_event_paranoid = <setting>) > > > > Hmm, I would rather like pointing manpages... > > Man pages are long, if you quote the relevant part of it when the > problem takes place, IMHO it helps the user.
OK, but please also update man pages (*), which can provide a summarized information for users if they noticed that. And for each place, I will add some messages for letting them know.
(*) https://lore.kernel.org/lkml/162204068898.388434.16842705842611255787.stgit@devnote2/
Thank you,
> > - Arnaldo > > > Would we better to have perf-security.7 manpage? > > > > Thank you, > > > > > $ > > > > > > - Arnaldo > > > > > > > > > > > > > > > > > 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> > > > > > > -- > > > > > > - Arnaldo > > > > > > -- > > Masami Hiramatsu <mhiramat@kernel.org> > > -- > > - Arnaldo
-- Masami Hiramatsu <mhiramat@kernel.org>
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