Commit 3debb0a9 authored Mar 22, 2016 by Steven Rostedt (Red Hat) Committed by Steven Rostedt Mar 22, 2016

tracing: Fix trace_printk() to print when not using bprintk()



The trace_printk() code will allocate extra buffers if the compile detects
that a trace_printk() is used. To do this, the format of the trace_printk()
is saved to the __trace_printk_fmt section, and if that section is bigger
than zero, the buffers are allocated (along with a message that this has
happened).

If trace_printk() uses a format that is not a constant, and thus something
not guaranteed to be around when the print happens, the compiler optimizes
the fmt out, as it is not used, and the __trace_printk_fmt section is not
filled. This means the kernel will not allocate the special buffers needed
for the trace_printk() and the trace_printk() will not write anything to the
tracing buffer.

Adding a "__used" to the variable in the __trace_printk_fmt section will
keep it around, even though it is set to NULL. This will keep the string
from being printed in the debugfs/tracing/printk_formats section as it is
not needed.

Reported-by: Vlastimil Babka <vbabka@suse.cz>
Fixes: 07d777fe "tracing: Add percpu buffers for trace_printk()"
Cc: stable@vger.kernel.org # v3.5+
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

parent 741f3a69

include/linux/kernel.h

+3 −3

Original line number	Diff line number	Diff line
		@@ -635,7 +635,7 @@ do { \

		#define do_trace_printk(fmt, args...) \
		do { \
		static const char *trace_printk_fmt \
		static const char *trace_printk_fmt __used \
		__attribute__((section("__trace_printk_fmt"))) = \
		__builtin_constant_p(fmt) ? fmt : NULL; \
		\
		@@ -679,7 +679,7 @@ int __trace_printk(unsigned long ip, const char *fmt, ...);
		*/

		#define trace_puts(str) ({ \
		static const char *trace_printk_fmt \
		static const char *trace_printk_fmt __used \
		__attribute__((section("__trace_printk_fmt"))) = \
		__builtin_constant_p(str) ? str : NULL; \
		\
		@@ -701,7 +701,7 @@ extern void trace_dump_stack(int skip);
		#define ftrace_vprintk(fmt, vargs) \
		do { \
		if (__builtin_constant_p(fmt)) { \
		static const char *trace_printk_fmt \
		static const char *trace_printk_fmt __used \
		__attribute__((section("__trace_printk_fmt"))) = \
		__builtin_constant_p(fmt) ? fmt : NULL; \
		\

kernel/trace/trace_printk.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -296,6 +296,9 @@ static int t_show(struct seq_file m, void v)
		const char str = fmt;
		int i;

		if (!*fmt)
		return 0;

		seq_printf(m, "0x%lx : \"", (unsigned long )fmt);

		/*

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