This is a note to let you know that I've just added the patch titled
tracing: Fix trace_printk() to print when not using bprintk()
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tracing-fix-trace_printk-to-print-when-not-using-bprintk.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 3debb0a9ddb16526de8b456491b7db60114f7b5e Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
Date: Tue, 22 Mar 2016 17:30:58 -0400
Subject: tracing: Fix trace_printk() to print when not using bprintk()
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: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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
@@ -607,7 +607,7 @@ 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; \
\
@@ -651,7 +651,7 @@ 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; \
\
@@ -673,7 +673,7 @@ 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
@@ -296,6 +296,9 @@ 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);
/*
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-4.4/tracing-fix-trace_printk-to-print-when-not-using-bprintk.patch
queue-4.4/tracing-fix-crash-from-reading-trace_pipe-with-sendfile.patch
queue-4.4/tracing-have-preempt-irqs-off-trace-preempt-disabled-functions.patch
--
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