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Subject[PATCH v2] perf ftrace: Fix the buffer size in __write_tracing_file
Date
From: Changbin Du <changbin.du@intel.com>

The terminal character '\0' should take into account into size of the
string buffer. Without this fix, the '--graph-funcs', '--nograph-funcs'
and '--trace-funcs' options didn't work as expected when the <func>
doesn't exist. If usersapce writes a non-terminated string, the kernel
side will always return success but actually no filter applied.

As discussed before, the kernel now support '\0' to mark the end of string:
https://lkml.org/lkml/2018/1/16/116

After this fix in userspace, the perf will report correct error state. Also
let it print an error if reset_tracing_files() fails.

The problem:
$ 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.

Signed-off-by: Changbin Du <changbin.du@intel.com>
---
tools/perf/builtin-ftrace.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index 25a42ac..a87e9b3 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -69,7 +69,7 @@ static int __write_tracing_file(const char *name, const char *val, bool append)
{
char *file;
int fd, ret = -1;
- ssize_t size = strlen(val);
+ ssize_t size = strlen(val) + 1;
int flags = O_WRONLY;
char errbuf[512];

@@ -280,8 +280,10 @@ 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
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