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bpf: Use dedicated bpf_trace_printk event instead of trace_printk()
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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) <>
Signed-off-by: Alan Maguire <>
Signed-off-by: Alexei Starovoitov <>
Acked-by: Andrii Nakryiko <>
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alan-maguire authored and Alexei Starovoitov committed Jul 13, 2020
1 parent 93776cb commit ac5a72e
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Showing 3 changed files with 73 additions and 5 deletions.
2 changes: 2 additions & 0 deletions kernel/trace/Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,8 @@ ifdef CONFIG_GCOV_PROFILE_FTRACE

CFLAGS_bpf_trace.o := -I$(src)

CFLAGS_trace_benchmark.o := -I$(src)
CFLAGS_trace_events_filter.o := -I$(src)

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42 changes: 37 additions & 5 deletions kernel/trace/bpf_trace.c
Original file line number Diff line number Diff line change
Expand Up @@ -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>
Expand All @@ -20,6 +21,9 @@
#include "trace_probe.h"
#include "trace.h"

#include "bpf_trace.h"

#define bpf_event_rcu_dereference(p) \
rcu_dereference_protected(p, lockdep_is_held(&bpf_event_mutex))

Expand Down Expand Up @@ -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);


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);
/* vsnprintf() will not append null for zero-length strings */
if (ret == 0)
buf[0] = '\0';
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
Expand Down Expand Up @@ -484,8 +512,7 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
#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)) \
Expand Down Expand Up @@ -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.
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;
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34 changes: 34 additions & 0 deletions kernel/trace/bpf_trace.h
Original file line number Diff line number Diff line change
@@ -0,0 +1,34 @@
/* SPDX-License-Identifier: GPL-2.0 */
#define TRACE_SYSTEM bpf_trace

#if !defined(_TRACE_BPF_TRACE_H) || defined(TRACE_HEADER_MULTI_READ)


#include <linux/tracepoint.h>


TP_PROTO(const char *bpf_string),


__string(bpf_string, bpf_string)

__assign_str(bpf_string, bpf_string);

TP_printk("%s", __get_str(bpf_string))

#endif /* _TRACE_BPF_TRACE_H */

#define TRACE_INCLUDE_FILE bpf_trace

#include <trace/define_trace.h>

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