[PATCH v4] perf ftrace: Append an EOL when write tracing files

[changbin.du@intel.com]

From: Changbin Du <changbin.du@intel.com>

Before this change, the '--graph-funcs', '--nograph-funcs' and
'--trace-funcs' options didn't work as expected when the <func> doesn't
exist. Because the kernel side hid possible errors.

$ sudo ./perf ftrace -a --graph-depth 1 --graph-funcs abcdefg
 0)   0.140 us    |  rcu_all_qs();
 3)   0.304 us    |  mutex_unlock();
 0)   0.153 us    |  find_vma();
 3)   0.088 us    |  __fsnotify_parent();
 0)   6.145 us    |  handle_mm_fault();
 3)   0.089 us    |  fsnotify();
 3)   0.161 us    |  __sb_end_write();
 3)   0.710 us    |  SyS_close();
 3)   7.848 us    |  exit_to_usermode_loop();

On above example, I specified function filter 'abcdefg' but all functions
are enabled. The expected error is hidden.

The original fix is to make the kernel support '\0' as end of string:
https://lkml.org/lkml/2018/1/16/116

But above fix cannot be compatible with old kernels. Then Namhyung Kim
suggest adding a space after function name.

This patch will append an '\n' when write tracing file. After this fix,
the perf will report correct error state. Also let it print an error if
reset_tracing_files() fails.

Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Changbin Du <changbin.du@intel.com>

---
v4: check strdup() return. (Kim)
v3: Took Kim's suggestion that add a space after function name.
v2: Rebase.
---
 tools/perf/builtin-ftrace.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index 25a42ac..f42f228 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -72,6 +72,7 @@ static int __write_tracing_file(const char *name, const char *val, bool append)
 	ssize_t size = strlen(val);
 	int flags = O_WRONLY;
 	char errbuf[512];
+	char *val_copy;
 
 	file = get_tracing_file(name);
 	if (!file) {
@@ -91,12 +92,23 @@ static int __write_tracing_file(const char *name, const char *val, bool append)
 		goto out;
 	}
 
-	if (write(fd, val, size) == size)
+	/*
+	 * Copy the original value and append a '\n'. Without this,
+	 * the kernel can hide possible errors.
+	 */
+	val_copy = strdup(val);
+	if (!val_copy)
+		goto out_close;
+	val_copy[size] = '\n';
+
+	if (write(fd, val_copy, size + 1) == size + 1)
 		ret = 0;
 	else
 		pr_debug("write '%s' to tracing/%s failed: %s\n",
 			 val, name, str_error_r(errno, errbuf, sizeof(errbuf)));
 
+	free(val_copy);
+out_close:
 	close(fd);
 out:
 	put_tracing_file(file);
@@ -280,8 +292,10 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
 	signal(SIGCHLD, sig_handler);
 	signal(SIGPIPE, sig_handler);
 
-	if (reset_tracing_files(ftrace) < 0)
+	if (reset_tracing_files(ftrace) < 0) {
+		pr_err("failed to reset ftrace\n");
 		goto out;
+	}
 
 	/* reset ftrace buffer */
 	if (write_tracing_file("trace", "0") < 0)
-- 
2.7.4