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Date: Tue, 22 Feb 2011 09:10:46 -0300
From: Arnaldo Carvalho de Melo <acme@...radead.org>
To: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc: Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
Frederic Weisbecker <fweisbec@...il.com>,
Mike Galbraith <efault@....de>,
Paul Mackerras <paulus@...ba.org>,
Peter Zijlstra <peterz@...radead.org>,
Stephane Eranian <eranian@...gle.com>,
Tom Zanussi <tzanussi@...il.com>,
"2nddept-manager@....hitachi.co.jp"
<2nddept-manager@....hitachi.co.jp>
Subject: Re: [PATCH 1/2] perf probe: Fix error propagation leading to
segfault
Em Tue, Feb 22, 2011 at 12:20:22PM +0900, Masami Hiramatsu escreveu:
> (2011/02/22 10:31), Arnaldo Carvalho de Melo wrote:
> > From: Arnaldo Carvalho de Melo <acme@...hat.com>
> > There are two hunks in this patch that stops probe processing as soon as one
> > error is found, breaking out of loops, the other fix an error propagation that
> > should return a negative error number but instead was returning the result of
> > "ret < 0", which is 1 and thus made several error checks fail because they test
> > agains < 0.
> >
> > The problem could be triggered by asking for a variable that was optimized out,
> > fact that should stop the whole probe processing but instead was segfaulting
> > while installing broken probes:
> >
> > [root@...lia ~]# probe perf_mmap:55 user_lock_limit
> > Failed to find the location of user_lock_limit at this address.
> > Perhaps, it has been optimized out.
> > Failed to find 'user_lock_limit' in this function.
> > Add new events:
> > probe:perf_mmap (on perf_mmap:55 with user_lock_limit)
> > probe:perf_mmap_1 (on perf_mmap:55 with user_lock_limit)
> > Segmentation fault (core dumped)
> > [root@...lia ~]# perf probe -l
> > probe:perf_mmap (on perf_mmap:55@.../linux/kernel/perf_event.c with user_lock_limit)
> > probe:perf_mmap_1 (on perf_mmap:55@.../linux/kernel/perf_event.c with user_lock_limit)
> > [root@...lia ~]#
> >
> > After the fix:
> >
> > [root@...lia ~]# probe perf_mmap:55 user_lock_limit
> > Failed to find the location of user_lock_limit at this address.
> > Perhaps, it has been optimized out.
> > Failed to find 'user_lock_limit' in this function.
> > Error: Failed to add events. (-2)
> > [root@...lia ~]#
>
> Oops, thanks! But I've also found this fix including some
> redundant checks.
I'll remove the redundancies as I describe later on this message.
> > Cc: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> > Cc: Mike Galbraith <efault@....de>
> > Cc: Paul Mackerras <paulus@...ba.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Stephane Eranian <eranian@...gle.com>
> > Cc: Tom Zanussi <tzanussi@...il.com>
> > LKML-Reference: <new-submission>
> > Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> > ---
> > tools/perf/util/probe-event.c | 5 ++++-
> > tools/perf/util/probe-finder.c | 4 +++-
> > 2 files changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> > index 0e3ea13..369ddc6 100644
> > --- a/tools/perf/util/probe-event.c
> > +++ b/tools/perf/util/probe-event.c
> > @@ -1832,9 +1832,12 @@ int add_perf_probe_events(struct perf_probe_event *pevs, int npevs,
> > }
> >
> > /* Loop 2: add all events */
> > - for (i = 0; i < npevs && ret >= 0; i++)
> > + for (i = 0; i < npevs && ret >= 0; i++) {
> > ret = __add_probe_trace_events(pkgs[i].pev, pkgs[i].tevs,
> > pkgs[i].ntevs, force_add);
> > + if (ret < 0)
> > + break;
> > + }
>
> Hmm, we've already checked ret >= 0 in for().
You see? There is value in sticking to common practices, even being one
line above I automatically looked after the assignment, not before. :-\
For that to work one needs to make sure to have ret initialized to zero
before the loop and do an unneeded test before we start the
__add_probe_trace_events calls.
> > end:
> > /* Loop 3: cleanup and free trace events */
> > for (i = 0; i < npevs; i++) {
> > diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
> > index fe461f6..eecbdca 100644
> > --- a/tools/perf/util/probe-finder.c
> > +++ b/tools/perf/util/probe-finder.c
> > @@ -1262,7 +1262,7 @@ static int probe_point_line_walker(const char *fname, int lineno,
> > ret = call_probe_finder(NULL, pf);
> >
> > /* Continue if no error, because the line will be in inline function */
> > - return ret < 0 ?: 0;
> > + return ret < 0 ? ret : 0;
>
> I think the problem is only here.
>
> > }
> >
> > /* Find probe point from its line number */
> > @@ -1484,6 +1484,8 @@ static int find_probes(int fd, struct probe_finder *pf)
> > pf->lno = pp->line;
> > ret = find_probe_point_by_line(pf);
> > }
> > + if (ret != DWARF_CB_OK)
> > + break;
>
> Actually, we must check that ret < 0 here, and that has been checked
> in while().
Ok, another instance of the above pet peeve of mine :-)
> > }
> > off = noff;
> > }
>
> Only with the second hunk of the patch, I've checked that is enough to fix
> the problem.
>
> $ ./perf probe -vv perf_mmap:55 user_lock_limit
> probe-definition(0): perf_mmap:55 user_lock_limit
> symbol:perf_mmap file:(null) line:55 offset:0 return:0 lazy:(null)
> parsing arg: user_lock_limit into user_lock_limit
> 1 arguments
> Looking at the vmlinux_path (6 entries long)
> Using //lib/modules/2.6.38-rc5-tip+/build/vmlinux for symbols
> Try to open /lib/modules/2.6.38-rc5-tip+/build/vmlinux
> Get 4514 lines from this CU
> Probe point found: perf_mmap+352
> Searching 'user_lock_limit' variable in context.
> Converting variable user_lock_limit into trace event.
> user_lock_limit type is long unsigned int.
> Probe point found: perf_mmap+359
> Searching 'user_lock_limit' variable in context.
> Converting variable user_lock_limit into trace event.
> user_lock_limit type is long unsigned int.
> Probe point found: perf_mmap+322
> Searching 'user_lock_limit' variable in context.
> Converting variable user_lock_limit into trace event.
> Failed to find the location of user_lock_limit at this address.
> Perhaps, it has been optimized out.
> Failed to find 'user_lock_limit' in this function.
> An error occurred in debuginfo analysis (-2).
> Error: Failed to add events. (-2)
Thanks for checking,
- Arnaldo
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