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

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@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>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 341551c7b4c82d265d6f511442459911206de7f3..5f4554b8348ad4c5fe8ce22563aa1602ff007ee0 100644 (file)
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -557,7 +557,7 @@ 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;                 \
                                                                        \
@@ -604,7 +604,7 @@ extern int __trace_puts(unsigned long ip, const char *str, int size);
  */
 
 #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;                 \
                                                                        \
@@ -624,7 +624,7 @@ 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 a9077c1b4ad3f402ecc7580b8c918e0b50db5318..fdb23e84b011b3ce5be6bc0f25498f782f01ec44 100644 (file)
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -272,6 +272,9 @@ 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);
 
        /*