bpf: Use dedicated bpf_trace_printk event instead of trace_printk()

The bpf helper bpf_trace_printk() uses trace_printk() under the hood.
This leads to an alarming warning message originating from trace
buffer allocation which occurs the first time a program using
bpf_trace_printk() is loaded.

We can instead create a trace event for bpf_trace_printk() and enable
it in-kernel when/if we encounter a program using the
bpf_trace_printk() helper.  With this approach, trace_printk()
is not used directly and no warning message appears.

This work was started by Steven (see Link) and finished by Alan; added
Steven's Signed-off-by with his permission.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/r/20200628194334.6238b933@oasis.local.home
Link: https://lore.kernel.org/bpf/1594641154-18897-2-git-send-email-alan.maguire@oracle.com

kernel/trace/Makefile

@@ -31,6 +31,8 @@ ifdef CONFIG_GCOV_PROFILE_FTRACE
 GCOV_PROFILE := y
 endif
 
+CFLAGS_bpf_trace.o := -I$(src)
+
 CFLAGS_trace_benchmark.o := -I$(src)
 CFLAGS_trace_events_filter.o := -I$(src)
 

kernel/trace/bpf_trace.c

@@ -11,6 +11,7 @@
 #include <linux/uaccess.h>
 #include <linux/ctype.h>
 #include <linux/kprobes.h>
+#include <linux/spinlock.h>
 #include <linux/syscalls.h>
 #include <linux/error-injection.h>
 #include <linux/btf_ids.h>
@@ -20,6 +21,9 @@
 #include "trace_probe.h"
 #include "trace.h"
 
+#define CREATE_TRACE_POINTS
+#include "bpf_trace.h"
+
 #define bpf_event_rcu_dereference(p)					\
 	rcu_dereference_protected(p, lockdep_is_held(&bpf_event_mutex))
 
@@ -375,6 +379,30 @@ static void bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
 	}
 }
 
+static DEFINE_RAW_SPINLOCK(trace_printk_lock);
+
+#define BPF_TRACE_PRINTK_SIZE   1024
+
+static inline __printf(1, 0) int bpf_do_trace_printk(const char *fmt, ...)
+{
+	static char buf[BPF_TRACE_PRINTK_SIZE];
+	unsigned long flags;
+	va_list ap;
+	int ret;
+
+	raw_spin_lock_irqsave(&trace_printk_lock, flags);
+	va_start(ap, fmt);
+	ret = vsnprintf(buf, sizeof(buf), fmt, ap);
+	va_end(ap);
+	/* vsnprintf() will not append null for zero-length strings */
+	if (ret == 0)
+		buf[0] = '\0';
+	trace_bpf_trace_printk(buf);
+	raw_spin_unlock_irqrestore(&trace_printk_lock, flags);
+
+	return ret;
+}
+
 /*
  * Only limited trace_printk() conversion specifiers allowed:
  * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %pB %pks %pus %s
@@ -484,8 +512,7 @@ fmt_next:
  */
 #define __BPF_TP_EMIT()	__BPF_ARG3_TP()
 #define __BPF_TP(...)							\
-	__trace_printk(0 /* Fake ip */,					\
-		       fmt, ##__VA_ARGS__)
+	bpf_do_trace_printk(fmt, ##__VA_ARGS__)
 
 #define __BPF_ARG1_TP(...)						\
 	((mod[0] == 2 || (mod[0] == 1 && __BITS_PER_LONG == 64))	\
@@ -522,10 +549,15 @@ static const struct bpf_func_proto bpf_trace_printk_proto = {
 const struct bpf_func_proto *bpf_get_trace_printk_proto(void)
 {
 	/*
-	 * this program might be calling bpf_trace_printk,
-	 * so allocate per-cpu printk buffers
+	 * This program might be calling bpf_trace_printk,
+	 * so enable the associated bpf_trace/bpf_trace_printk event.
+	 * Repeat this each time as it is possible a user has
+	 * disabled bpf_trace_printk events.  By loading a program
+	 * calling bpf_trace_printk() however the user has expressed
+	 * the intent to see such events.
 	 */
-	trace_printk_init_buffers();
+	if (trace_set_clr_event("bpf_trace", "bpf_trace_printk", 1))
+		pr_warn_ratelimited("could not enable bpf_trace_printk events");
 
 	return &bpf_trace_printk_proto;
 }

kernel/trace/bpf_trace.h

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM bpf_trace
+
+#if !defined(_TRACE_BPF_TRACE_H) || defined(TRACE_HEADER_MULTI_READ)
+
+#define _TRACE_BPF_TRACE_H
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(bpf_trace_printk,
+
+	TP_PROTO(const char *bpf_string),
+
+	TP_ARGS(bpf_string),
+
+	TP_STRUCT__entry(
+		__string(bpf_string, bpf_string)
+	),
+
+	TP_fast_assign(
+		__assign_str(bpf_string, bpf_string);
+	),
+
+	TP_printk("%s", __get_str(bpf_string))
+);
+
+#endif /* _TRACE_BPF_TRACE_H */
+
+#undef TRACE_INCLUDE_PATH
+#define TRACE_INCLUDE_PATH .
+#define TRACE_INCLUDE_FILE bpf_trace
+
+#include <trace/define_trace.h>