[PATCH 3.16 135/217] tracing: Fix trace_printk() to print when not using bprintk()

From: Ben Hutchings
Date: Tue Apr 26 2016 - 20:09:49 EST


3.16.35-rc1 review patch. If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

commit 3debb0a9ddb16526de8b456491b7db60114f7b5e upstream.

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@xxxxxxx>
Fixes: 07d777fe8c398 "tracing: Add percpu buffers for trace_printk()"
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
include/linux/kernel.h | 6 +++---
kernel/trace/trace_printk.c | 3 +++
2 files changed, 6 insertions(+), 3 deletions(-)

--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -596,7 +596,7 @@ 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; \
\
@@ -640,7 +640,7 @@ int __trace_printk(unsigned long ip, con
*/

#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; \
\
@@ -662,7 +662,7 @@ 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; \
\
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -292,6 +292,9 @@ static int t_show(struct seq_file *m, vo
const char *str = *fmt;
int i;

+ if (!*fmt)
+ return 0;
+
seq_printf(m, "0x%lx : \"", *(unsigned long *)fmt);

/*