In the past, I've had lots of reports about trace events not working.
Developers would say they put a trace_printk() before and after the trace
event but when they enable it (and the trace event said it was enabled) they
would see the trace_printks but not the trace event.
I was not able to reproduce this, but that's because I wasn't looking at
the right location. Recently, another bug came up that showed the issue.
If your kernel supports signed modules but allows for non-signed modules
to be loaded, then when one is, the kernel will silently set the
MODULE_FORCED taint on the module. Although, this taint happens without
the need for insmod --force or anything of the kind, it labels the
module with that taint anyway.
If this tainted module has tracepoints, the tracepoints will be ignored
because of the MODULE_FORCED taint. But no error message will be
displayed. Worse yet, the event infrastructure will still be created
letting users enable the trace event represented by the tracepoint,
although that event will never actually be enabled. This is because
the tracepoint infrastructure allows for non-existing tracepoints to
be enabled for new modules to arrive and have their tracepoints set.
Although there are several things wrong with the above, this change
only addresses the creation of the trace event files for tracepoints
that are not created when a module is loaded and is tainted. This change
will print an error message about the module being tainted and not the
trace events will not be created, and it does not create the trace event
infrastructure.
tracing: Do not add event files for modules that fail tracepoints
If a module fails to add its tracepoints due to module tainting, do not
create the module event infrastructure in the debugfs directory. As the events
will not work and worse yet, they will silently fail, making the user wonder
why the events they enable do not display anything.
Having a warning on module load and the events not visible to the users
will make the cause of the problem much clearer.
Link: http://lkml.kernel.org/r/20140227154923.265882695@goodmis.org
Fixes: 6d723736e472 "tracing/events: add support for modules to TRACE_EVENT"
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org # 2.6.31+
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
3 files changed
