[PATCH 1/1] tracing: Fix event_trace_printk loss on printk_format.

From: Mars Cheng
Date: Tue Feb 12 2019 - 06:42:14 EST

From: Andress Kuo <andress.kuo@xxxxxxxxxxxx>

If fmt on event_trace_printk is not a constant, It means that something not
guaranteed, so the compiler optimizes the fmt out, and then the
__trace_printk_fmt section is not filled. if __trace_printk_fmt is not
filled, the kernel will not allocate the special buffers needed for the
event_trace_printk() and then not shown in the file output

Adding a "__used" to the variable in the __trace_printk_fmt section on
event_trace_printk() will keep it around, even though it is set to NULL.
This will keep the string from being printed in the
sys/kernel/debug/tracing/printk_formats section.

We can also refer to commit 3debb0a9ddb1 ("tracing: Fix trace_printk()
to print when not using bprintk()")that it had similar issue on path of

Signed-off-by: Andress Kuo <andress.kuo@xxxxxxxxxxxx>
Signed-off-by: Mars Cheng <mars.cheng@xxxxxxxxxxxx>
include/linux/trace_events.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 8a62731..26bd7c4 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -551,7 +551,7 @@ extern int trace_define_field(struct trace_event_call *call, const char *type,
__trace_printk_check_format(fmt, ##args); \
tracing_record_cmdline(current); \
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; \