[for-next][PATCH 3/7] tracing: Print nasty banner when trace_printk() is in use

Steven Rostedt
2014-05-31T01:39:28Z

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From: Steven Rostedt <rostedt@goodmis.org>

trace_printk() is used to debug fast paths within the kernel. Places
that gets called in any context (interrupt or NMI) or thousands of
times a second. Something you do not want to do with a printk().

In order to make it completely lockless as it needs a temporary buffer
to handle some of the string formatting, a page is created per cpu for
every context (four per cpu; normal, softirq, irq, NMI).

Since trace_printk() should only be used for debugging purposes,
there's no reason to waste memory on these buffers on a production
system. That means, trace_printk() should never be used unless a
developer is debugging their kernel. There's macro magic to allocate
the buffers if trace_printk() is used anywhere in the kernel.

To help enforce that trace_printk() isn't used outside of development,
when it is used, a nasty banner is displayed on bootup (or when a module
is loaded that uses trace_printk() and the kernel core does not).

Here's the banner:

**********************************************************
** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **
** **
** trace_printk() being used. Allocating extra memory. **
** **
** This means that this is a DEBUG kernel and it is **
** unsafe for produciton use. **
** **
** If you see this message and you are not debugging **
** the kernel, report this immediately to your vendor! **
** **
** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **
**********************************************************

That should hopefully keep developers from trying to sneak in a
trace_printk() or two.

Link: http://lkml.kernel.org/p/20140528131440.2283213c@gandalf.local.home

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/trace/trace.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 05431696b10c..eb228b9de170 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1975,7 +1975,21 @@ void trace_printk_init_buffers(void)
if (alloc_percpu_trace_buffer())
return;

- pr_info("ftrace: Allocated trace_printk buffers\n");
+ /* trace_printk() is for debug use only. Don't use it in production. */
+
+ pr_warning("\n**********************************************************\n");
+ pr_warning("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
+ pr_warning("** **\n");
+ pr_warning("** trace_printk() being used. Allocating extra memory. **\n");
+ pr_warning("** **\n");
+ pr_warning("** This means that this is a DEBUG kernel and it is **\n");
+ pr_warning("** unsafe for produciton use. **\n");
+ pr_warning("** **\n");
+ pr_warning("** If you see this message and you are not debugging **\n");
+ pr_warning("** the kernel, report this immediately to your vendor! **\n");
+ pr_warning("** **\n");
+ pr_warning("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
+ pr_warning("**********************************************************\n");

/* Expand the buffers to set size */
tracing_update_buffers();
--
2.0.0.rc2