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: 07d777fe8c398 "tracing: Add percpu buffers for trace_printk()"
Cc: stable@vger.kernel.org # v3.5+
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index f31638c6e8738230edebe2606ff27d8782fc1a19..95452f72349adba4d36e6ccf0b5e2600ee03bfe5 100644 (file)
--- 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 060df67dbdd1d038dba8ec35909890f3e892b6d7..f96f0383f6c6bfe40cba9226b97f8885723a4d9d 100644 (file)
--- 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);
 
        /*