Re: [PATCH 1/2] perf tools: Fix seg fault with Intel PT

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Jan 26, 2016 at 03:34:15PM +0200, Adrian Hunter escreveu:
> On 26/01/16 15:23, Arnaldo Carvalho de Melo wrote:
> > Em Tue, Jan 26, 2016 at 02:05:20PM +0200, Adrian Hunter escreveu:
> >> Fix segmentation fault using:
> >>
> >> 	perf record -e intel_pt//u uname
> >>
> >> Back trace:
> >>
> >>   0  tracepoint_error (err=13, err@entry=-13, sys=sys@entry=0x18706c0 "sched", name=name@entry=0x1871c70 "sched_switch", e=<optimized out>, e=<optimized out>) at util/parse-events.c:416
> >>   1  0x000000000049791c in add_tracepoint (head_config=0x0, err=0x0, evt_name=0x1871c70 "sched_switch", sys_name=0x18706c0 "sched", idx=<optimized out>, list=<optimized out>)
> >>      at util/parse-events.c:433
> >>   2  add_tracepoint_event (list=<optimized out>, idx=<optimized out>, sys_name=0x18706c0 "sched", evt_name=0x1871c70 "sched_switch", err=0x0, head_config=0x0) at util/parse-events.c:498
> >>   3  0x0000000000498ec4 in parse_events_add_tracepoint (list=list@entry=0x1871c90, idx=idx@entry=0x7fffffffba30, sys=0x18706c0 "sched", event=0x1871c70 "sched_switch", err=err@entry=0x0,
> >>      head_config=head_config@entry=0x0) at util/parse-events.c:936
> >>   4  0x00000000004ccff2 in parse_events_parse (_data=_data@entry=0x7fffffffba20, scanner=0x18705f0) at util/parse-events.y:391
> >>   5  0x000000000049a567 in parse_events__scanner (start_token=258, data=0x7fffffffba20, str=0x0) at util/parse-events.c:1361
> >>   6  parse_events (evlist=evlist@entry=0x1871150, str=str@entry=0x55ef6c "sched:sched_switch", err=err@entry=0x0) at util/parse-events.c:1401
> >>   7  0x00000000004eb4b6 in perf_evlist__can_select_event (evlist=evlist@entry=0x186fb90, str=str@entry=0x55ef6c "sched:sched_switch") at util/record.c:253
> >>   8  0x000000000051e1d3 in intel_pt_track_switches (evlist=0x186fb90) at arch/x86/util/intel-pt.c:362
> >>   9  intel_pt_recording_options (itr=0x186ef70, evlist=0x186fb90, opts=0x7c3e08 <record+232>) at arch/x86/util/intel-pt.c:662
> >>  10  0x000000000042e244 in cmd_record (argc=1, argv=0x7fffffffe270, prefix=<optimized out>) at builtin-record.c:1260
> >>  11  0x000000000047cbd3 in run_builtin (p=p@entry=0x7cf2e8 <commands+168>, argc=argc@entry=4, argv=argv@entry=0x7fffffffe270) at perf.c:390
> >>  12  0x00000000004216a7 in handle_internal_command (argv=0x7fffffffe270, argc=4) at perf.c:451
> >>  13  run_argv (argv=0x7fffffffdff0, argcp=0x7fffffffdffc) at perf.c:495
> >>  14  main (argc=4, argv=0x7fffffffe270) at perf.c:618
> >>
> >> Intel PT attempts to find the sched:sched_switch tracepoint but that
> >> seg faults if tracefs is not readable, because the error reporting
> > 
> > How can I reproduce this problem? I tried your command but that doesn't cause
> > any problem:
> > 
> > [acme@jouet linux]$ ls -la /sys/kernel/debug/tracing/
> > ls: cannot access /sys/kernel/debug/tracing/: Permission denied
> > [acme@jouet linux]$ ls -la /sys/kernel/debug/
> > ls: cannot open directory /sys/kernel/debug/: Permission denied
> > [acme@jouet linux]$ perf record -e intel_pt//u uname
> > Linux
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 0.041 MB perf.data ]
> > [acme@jouet linux]$ perf list intel
> > 
> > List of pre-defined events (to be used in -e):
> > 
> >   intel_bts//                                        [Kernel PMU event]
> >   intel_pt//                                         [Kernel PMU event]
> > 
> > [acme@jouet linux]$ perf record -e intel_pt//u uname
> > Linux
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 0.041 MB perf.data ]
> > [acme@jouet linux]$ 
> > 
> > [acme@jouet linux]$ perf evlist
> > intel_pt//u
> > dummy:u
> > [acme@jouet linux]$ 
> > 
> > [acme@jouet linux]$ perf evlist -v
> > intel_pt//u: type: 6, size: 112, config: 0x400, { sample_period, sample_freq }: 1, sample_type: IP|TID|TIME|CPU|IDENTIFIER, read_format: ID, disabled: 1, inherit: 1, exclude_kernel: 1, exclude_hv: 1, enable_on_exec: 1, sample_id_all: 1
> > dummy:u: type: 1, size: 112, config: 0x9, { sample_period, sample_freq }: 1, sample_type: IP|TID|TIME|CPU|IDENTIFIER, read_format: ID, disabled: 1, inherit: 1, exclude_kernel: 1, exclude_hv: 1, mmap: 1, comm: 1, enable_on_exec: 1, task: 1, sample_id_all: 1, mmap2: 1, comm_exec: 1, context_switch: 1
> > [acme@jouet linux]$ 
> > 
> > Humm, ok, so this happens only if tracefs is not readable _and_ this is an
> > older kernel where perf_event_attr.context_switch is not available?
> 
> Yes, I should have thought of that, sorry.

So, I just tried on another machine, with an older kernel
 
[acme@ssdandy linux]$ uname -r
3.10.0-229.14.1.el7.x86_64
[acme@ssdandy linux]$ ls -la /sys/kernel/debug/
ls: cannot open directory /sys/kernel/debug/: Permission denied
[acme@ssdandy linux]$ 
[acme@ssdandy linux]$ grep tracefs /proc/filesystems 
[acme@ssdandy linux]$ grep debugfs /proc/filesystems 
nodev	debugfs
[acme@ssdandy linux]$ ls -la /sys/kernel/debug/
ls: cannot open directory /sys/kernel/debug/: Permission denied
[acme@ssdandy linux]$ perf record -e intel_pt//u uname
invalid or unsupported event: 'intel_pt//u'
Run 'perf list' for a list of valid events

 Usage: perf record [<options>] [<command>]
    or: perf record [<options>] -- <command> [<options>]

    -e, --event <event>   event selector. use 'perf list' to list available events
[acme@ssdandy linux]$ 

So, this needs:

1) A machine that supports Intel PT
2) A kernel that supports Intel PT
3) A kernel that doesn't have perf_event_attr.context_switch, so that it tries
   to use sched:sched_switch
4) an unreadable tracefs

What was the kernel where you stumbled on this problem? What machine?

- Arnaldo

> > /me tries booting another machine, with an older kernel...
> > 
> >> structure is null, as errors are not reported when automatically
> >> adding tracepoints.  Fix by checking before using.
> >>
> >> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> >> Fixes: 196581717d85 ("perf tools: Enhance parsing events tracepoint error output")
> >> Cc: stable@vger.kernel.org # v4.4+
> >> ---
> >>  tools/perf/util/parse-events.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/tools/perf/util/parse-events.c b/tools/perf/util/parse-events.c
> >> index 4f7b0efdde2f..813d9b272c81 100644
> >> --- a/tools/perf/util/parse-events.c
> >> +++ b/tools/perf/util/parse-events.c
> >> @@ -399,6 +399,9 @@ static void tracepoint_error(struct parse_events_error *e, int err,
> >>  {
> >>  	char help[BUFSIZ];
> >>  
> >> +	if (!e)
> >> +		return;
> >> +
> >>  	/*
> >>  	 * We get error directly from syscall errno ( > 0),
> >>  	 * or from encoded pointer's error ( < 0).
> >> -- 
> >> 1.9.1
> >