Messages in this thread | | | Subject | Re: [PATCH v1] perf synthetic-events: Return error if procfs isn't mounted for PID namespaces | From | James Clark <> | Date | Fri, 7 Jan 2022 14:27:57 +0000 |
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On 24/12/2021 12:40, Leo Yan wrote: > For perf recording, it retrieves process info by iterating nodes in proc > fs. If we run perf in a non-root PID namespace with command: > > # unshare --fork --pid perf record -e cycles -a -- test_program > > ... in this case, unshare command creates a child PID namespace and > launches perf tool in it, but the issue is the proc fs is not mounted > for the non-root PID namespace, this leads to the perf tool gathering > process info from its parent PID namespace. >
I had some concerns that this would prevent use of perf in docker, but docker does mount /proc so it is still working. And you've added a warning about how to fix the unshare command so I think this is ok.
Reviewed-by: James Clark <james.clark@arm.com>
> We can use below command to observe the process nodes under proc fs: > > # unshare --pid --fork ls /proc > 1 137 1968 2128 3 342 48 62 78 crypto kcore net uptime > 10 138 2 2142 30 35 49 63 8 devices keys pagetypeinfo version > 11 139 20 2143 304 36 50 64 82 device-tree key-users partitions vmallocinfo > 12 14 2011 22 305 37 51 65 83 diskstats kmsg self vmstat > 128 140 2038 23 307 39 52 656 84 driver kpagecgroup slabinfo zoneinfo > 129 15 2074 24 309 4 53 67 9 execdomains kpagecount softirqs > 13 16 2094 241 31 40 54 68 asound fb kpageflags stat > 130 164 2096 242 310 41 55 69 buddyinfo filesystems loadavg swaps > 131 17 2098 25 317 42 56 70 bus fs locks sys > 132 175 21 26 32 43 57 71 cgroups interrupts meminfo sysrq-trigger > 133 179 2102 263 329 44 58 75 cmdline iomem misc sysvipc > 134 1875 2103 27 330 45 59 76 config.gz ioports modules thread-self > 135 19 2117 29 333 46 6 77 consoles irq mounts timer_list > 136 1941 2121 298 34 47 60 773 cpuinfo kallsyms mtd tty > > So it shows many existed tasks, since unshared command has not mounted > the proc fs for the new created PID namespace, it still accesses the > proc fs of the root PID namespace. This leads to two prominent issues: > > - Firstly, PID values are mismatched between thread info and samples. > The gathered thread info are coming from the proc fs of the root PID > namespace, but samples record its PID from the child PID namespace. > > - The second issue is profiled program 'test_program' returns its forked > PID number from the child PID namespace, perf tool wrongly uses this > PID number to retrieve the process info via the proc fs of the root > PID namespace. > > To avoid issues, we need to mount proc fs for the child PID namespace > with the option '--mount-proc' when use unshare command: > > # unshare --fork --pid --mount-proc perf record -e cycles -a -- test_program > > Conversely, when the proc fs of the root PID namespace is used by child > namespace, perf tool can detect the multiple PID levels and > nsinfo__is_in_root_namespace() returns false, this patch reports error > for this case: > > # unshare --fork --pid perf record -e cycles -a -- test_program > Couldn't synthesize bpf events. > Perf runs in non-root PID namespace but it tries to gather process info from its parent PID namespace. > Please mount the proc file system properly, e.g. add the option '--mount-proc' for unshare command. > > Signed-off-by: Leo Yan <leo.yan@linaro.org> > --- > tools/perf/util/synthetic-events.c | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c > index 198982109f0f..cf7800347f77 100644 > --- a/tools/perf/util/synthetic-events.c > +++ b/tools/perf/util/synthetic-events.c > @@ -1784,6 +1784,25 @@ int __machine__synthesize_threads(struct machine *machine, struct perf_tool *too > perf_event__handler_t process, bool needs_mmap, > bool data_mmap, unsigned int nr_threads_synthesize) > { > + /* > + * When perf runs in non-root PID namespace, and the namespace's proc FS > + * is not mounted, nsinfo__is_in_root_namespace() returns false. > + * In this case, the proc FS is coming for the parent namespace, thus > + * perf tool will wrongly gather process info from its parent PID > + * namespace. > + * > + * To avoid the confusion that the perf tool runs in a child PID > + * namespace but it synthesizes thread info from its parent PID > + * namespace, returns failure with warning. > + */ > + if (!nsinfo__is_in_root_namespace()) { > + pr_err("Perf runs in non-root PID namespace but it tries to "); > + pr_err("gather process info from its parent PID namespace.\n"); > + pr_err("Please mount the proc file system properly, e.g. "); > + pr_err("add the option '--mount-proc' for unshare command.\n"); > + return -EPERM; > + } > + > if (target__has_task(target)) > return perf_event__synthesize_thread_map(tool, threads, process, machine, > needs_mmap, data_mmap); >
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