tools: bpftool: add a command to dump the trace pipe

BPF programs can use the bpf_trace_printk() helper to print debug
information into the trace pipe. Add a subcommand
"bpftool prog tracelog" to simply dump this pipe to the console.

This is for a good part copied from iproute2, where the feature is
available with "tc exec bpf dbg". Changes include dumping pipe content
to stdout instead of stderr and adding JSON support (content is dumped
as an array of strings, one per line read from the pipe). This version
is dual-licensed, with Daniel's permission.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/Documentation/bpftool-prog.rst

@@ -26,8 +26,9 @@ MAP COMMANDS
 |	**bpftool** **prog dump jited**  *PROG* [{**file** *FILE* | **opcodes**}]
 |	**bpftool** **prog pin** *PROG* *FILE*
 |	**bpftool** **prog { load | loadall }** *OBJ* *PATH* [**type** *TYPE*] [**map** {**idx** *IDX* | **name** *NAME*} *MAP*] [**dev** *NAME*]
-|       **bpftool** **prog attach** *PROG* *ATTACH_TYPE* [*MAP*]
+|	**bpftool** **prog attach** *PROG* *ATTACH_TYPE* [*MAP*]
-|       **bpftool** **prog detach** *PROG* *ATTACH_TYPE* [*MAP*]
+|	**bpftool** **prog detach** *PROG* *ATTACH_TYPE* [*MAP*]
+|	**bpftool** **prog tracelog**
 |	**bpftool** **prog help**
 |
 |	*MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
@@ -117,6 +118,14 @@ DESCRIPTION
 		  parameter, with the exception of *flow_dissector* which is
 		  detached from the current networking name space.
 
+	**bpftool prog tracelog**
+		  Dump the trace pipe of the system to the console (stdout).
+		  Hit <Ctrl+C> to stop printing. BPF programs can write to this
+		  trace pipe at runtime with the **bpf_trace_printk()** helper.
+		  This should be used only for debugging purposes. For
+		  streaming data from BPF programs to user space, one can use
+		  perf events (see also **bpftool-map**\ (8)).
+
 	**bpftool prog help**
 		  Print short help message.
 

tools/bpf/bpftool/bash-completion/bpftool

@@ -398,10 +398,13 @@ _bpftool()
                             ;;
                     esac
                     ;;
+                tracelog)
+                    return 0
+                    ;;
                 *)
                     [[ $prev == $object ]] && \
                         COMPREPLY=( $( compgen -W 'dump help pin attach detach load \
-                            show list' -- "$cur" ) )
+                            show list tracelog' -- "$cur" ) )
                     ;;
             esac
             ;;

tools/bpf/bpftool/main.h

@@ -167,6 +167,7 @@ int do_event_pipe(int argc, char **argv);
 int do_cgroup(int argc, char **arg);
 int do_perf(int argc, char **arg);
 int do_net(int argc, char **arg);
+int do_tracelog(int argc, char **arg);
 
 int parse_u32_arg(int *argc, char ***argv, __u32 *val, const char *what);
 int prog_parse_fd(int *argc, char ***argv);

tools/bpf/bpftool/prog.c

@@ -1140,6 +1140,7 @@ static int do_help(int argc, char **argv)
 		"                         [pinmaps MAP_DIR]\n"
 		"       %s %s attach PROG ATTACH_TYPE [MAP]\n"
 		"       %s %s detach PROG ATTACH_TYPE [MAP]\n"
+		"       %s %s tracelog\n"
 		"       %s %s help\n"
 		"\n"
 		"       " HELP_SPEC_MAP "\n"
@@ -1158,7 +1159,7 @@ static int do_help(int argc, char **argv)
 		"",
 		bin_name, argv[-2], bin_name, argv[-2], bin_name, argv[-2],
 		bin_name, argv[-2], bin_name, argv[-2], bin_name, argv[-2],
-		bin_name, argv[-2], bin_name, argv[-2]);
+		bin_name, argv[-2], bin_name, argv[-2], bin_name, argv[-2]);
 
 	return 0;
 }
@@ -1173,6 +1174,7 @@ static const struct cmd cmds[] = {
 	{ "loadall",	do_loadall },
 	{ "attach",	do_attach },
 	{ "detach",	do_detach },
+	{ "tracelog",	do_tracelog },
 	{ 0 }
 };
 

tools/bpf/bpftool/tracelog.c

@@ -0,0 +1,157 @@
+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+/* Copyright (c) 2015-2017 Daniel Borkmann */
+/* Copyright (c) 2018 Netronome Systems, Inc. */
+
+#include <errno.h>
+#include <limits.h>
+#include <signal.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <linux/magic.h>
+#include <sys/fcntl.h>
+#include <sys/vfs.h>
+
+#include "main.h"
+
+#ifndef TRACEFS_MAGIC
+# define TRACEFS_MAGIC	0x74726163
+#endif
+
+#define _textify(x)	#x
+#define textify(x)	_textify(x)
+
+FILE *trace_pipe_fd;
+char *buff;
+
+static int validate_tracefs_mnt(const char *mnt, unsigned long magic)
+{
+	struct statfs st_fs;
+
+	if (statfs(mnt, &st_fs) < 0)
+		return -ENOENT;
+	if ((unsigned long)st_fs.f_type != magic)
+		return -ENOENT;
+
+	return 0;
+}
+
+static bool
+find_tracefs_mnt_single(unsigned long magic, char *mnt, const char *mntpt)
+{
+	size_t src_len;
+
+	if (validate_tracefs_mnt(mntpt, magic))
+		return false;
+
+	src_len = strlen(mntpt);
+	if (src_len + 1 >= PATH_MAX) {
+		p_err("tracefs mount point name too long");
+		return false;
+	}
+
+	strcpy(mnt, mntpt);
+	return true;
+}
+
+static bool find_tracefs_pipe(char *mnt)
+{
+	static const char * const known_mnts[] = {
+		"/sys/kernel/debug/tracing",
+		"/sys/kernel/tracing",
+		"/tracing",
+		"/trace",
+	};
+	const char *pipe_name = "/trace_pipe";
+	const char *fstype = "tracefs";
+	char type[100], format[32];
+	const char * const *ptr;
+	bool found = false;
+	FILE *fp;
+
+	for (ptr = known_mnts; ptr < known_mnts + ARRAY_SIZE(known_mnts); ptr++)
+		if (find_tracefs_mnt_single(TRACEFS_MAGIC, mnt, *ptr))
+			goto exit_found;
+
+	fp = fopen("/proc/mounts", "r");
+	if (!fp)
+		return false;
+
+	/* Allow room for NULL terminating byte and pipe file name */
+	snprintf(format, sizeof(format), "%%*s %%%zds %%99s %%*s %%*d %%*d\\n",
+		 PATH_MAX - strlen(pipe_name) - 1);
+	while (fscanf(fp, format, mnt, type) == 2)
+		if (strcmp(type, fstype) == 0) {
+			found = true;
+			break;
+		}
+	fclose(fp);
+
+	/* The string from fscanf() might be truncated, check mnt is valid */
+	if (!found || validate_tracefs_mnt(mnt, TRACEFS_MAGIC))
+		return false;
+
+exit_found:
+	strcat(mnt, pipe_name);
+	return true;
+}
+
+static void exit_tracelog(int signum)
+{
+	fclose(trace_pipe_fd);
+	free(buff);
+
+	if (json_output) {
+		jsonw_end_array(json_wtr);
+		jsonw_destroy(&json_wtr);
+	}
+
+	exit(0);
+}
+
+int do_tracelog(int argc, char **argv)
+{
+	const struct sigaction act = {
+		.sa_handler = exit_tracelog
+	};
+	char trace_pipe[PATH_MAX];
+	bool found_trace_pipe;
+	size_t buff_len = 0;
+
+	if (json_output)
+		jsonw_start_array(json_wtr);
+
+	found_trace_pipe = find_tracefs_pipe(trace_pipe);
+	if (!found_trace_pipe) {
+		p_err("could not find trace pipe, tracefs not mounted?");
+		return -1;
+	}
+
+	trace_pipe_fd = fopen(trace_pipe, "r");
+	if (!trace_pipe_fd) {
+		p_err("could not open trace pipe: %s", strerror(errno));
+		return -1;
+	}
+
+	sigaction(SIGHUP, &act, NULL);
+	sigaction(SIGINT, &act, NULL);
+	sigaction(SIGTERM, &act, NULL);
+	while (1) {
+		ssize_t ret;
+
+		ret = getline(&buff, &buff_len, trace_pipe_fd);
+		if (ret <= 0) {
+			p_err("failed to read content from trace pipe: %s",
+			      strerror(errno));
+			break;
+		}
+		if (json_output)
+			jsonw_string(json_wtr, buff);
+		else
+			printf("%s", buff);
+	}
+
+	fclose(trace_pipe_fd);
+	free(buff);
+	return -1;
+}