From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
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()"
Cc: stable@xxxxxxxxxxxxxxx # v3.5+
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
include/linux/kernel.h | 6 +++---
kernel/trace/trace_printk.c | 3 +++
2 files changed, 6 insertions(+), 3 deletions(-)
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index f31638c6e873..95452f72349a 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -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; \
\
diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
index 060df67dbdd1..f96f0383f6c6 100644
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -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);
/*
--
2.7.0
--
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