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Subject[PATCH v2] tracing/events: fix concurrent access to ftrace_events list
A module will add/remove its trace events when it gets loaded/unloaded, so
the ftrace_events list is not "const", and concurrent access needs to be
protected.

This patch thus fixes races between loading/unloding modules and read
'available_events' or read/write 'set_event', etc.

Below shows how to reproduce the race:

# for ((; ;)) { cat /mnt/tracing/available_events; } > /dev/null &
# for ((; ;)) { insmod trace-events-sample.ko; rmmod sample; } &
After a while:

BUG: unable to handle kernel paging request at 0010011c
IP: [<c1080f27>] t_next+0x1b/0x2d
...
Call Trace:
[<c10c90e6>] ? seq_read+0x217/0x30d
[<c10c8ecf>] ? seq_read+0x0/0x30d
[<c10b4c19>] ? vfs_read+0x8f/0x136
[<c10b4fc3>] ? sys_read+0x40/0x65
[<c1002a68>] ? sysenter_do_call+0x12/0x36

Changelog v1 -> v2:
- Fix deadlock when writing invalid pred into subsystem filter.

[ Impact: fix races when concurrent accessing ftrace_events list ]

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
kernel/trace/trace.h | 1 +
kernel/trace/trace_event_profile.c | 19 ++++++++++++++-----
kernel/trace/trace_events.c | 20 +++++++++++---------
kernel/trace/trace_events_filter.c | 5 +++++
4 files changed, 31 insertions(+), 14 deletions(-)
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 83984ab..0bba080 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -827,6 +827,7 @@ static int filter_pred_##size(struct filter_pred *pred, void *event, \
return match; \
}

+extern struct mutex event_mutex;
extern struct list_head ftrace_events;

extern const char *__start___trace_bprintk_fmt[];
diff --git a/kernel/trace/trace_event_profile.c b/kernel/trace/trace_event_profile.c
index 7bf2ad6..5b5895a 100644
--- a/kernel/trace/trace_event_profile.c
+++ b/kernel/trace/trace_event_profile.c
@@ -10,21 +10,30 @@
int ftrace_profile_enable(int event_id)
{
struct ftrace_event_call *event;
+ int ret = -EINVAL;

+ mutex_lock(&event_mutex);
list_for_each_entry(event, &ftrace_events, list) {
- if (event->id == event_id)
- return event->profile_enable(event);
+ if (event->id == event_id) {
+ ret = event->profile_enable(event);
+ break;
+ }
}
+ mutex_unlock(&event_mutex);

- return -EINVAL;
+ return ret;
}

void ftrace_profile_disable(int event_id)
{
struct ftrace_event_call *event;

+ mutex_lock(&event_mutex);
list_for_each_entry(event, &ftrace_events, list) {
- if (event->id == event_id)
- return event->profile_disable(event);
+ if (event->id == event_id) {
+ event->profile_disable(event);
+ break;
+ }
}
+ mutex_unlock(&event_mutex);
}
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 72d619d..ac5b1c2 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -21,7 +21,7 @@

#define TRACE_SYSTEM "TRACE_SYSTEM"

-static DEFINE_MUTEX(event_mutex);
+DEFINE_MUTEX(event_mutex);

LIST_HEAD(ftrace_events);

@@ -80,6 +80,7 @@ static void ftrace_clear_events(void)
{
struct ftrace_event_call *call;

+ mutex_lock(&event_mutex);
list_for_each_entry(call, &ftrace_events, list) {

if (call->enabled) {
@@ -87,6 +88,7 @@ static void ftrace_clear_events(void)
call->unregfunc();
}
}
+ mutex_unlock(&event_mutex);
}

static void ftrace_event_enable_disable(struct ftrace_event_call *call,
@@ -274,6 +276,9 @@ t_next(struct seq_file *m, void *v, loff_t *pos)

static void *t_start(struct seq_file *m, loff_t *pos)
{
+ mutex_lock(&event_mutex);
+ if (*pos == 0)
+ m->private = ftrace_events.next;
return t_next(m, NULL, pos);
}

@@ -303,6 +308,9 @@ s_next(struct seq_file *m, void *v, loff_t *pos)

static void *s_start(struct seq_file *m, loff_t *pos)
{
+ mutex_lock(&event_mutex);
+ if (*pos == 0)
+ m->private = ftrace_events.next;
return s_next(m, NULL, pos);
}

@@ -319,12 +327,12 @@ static int t_show(struct seq_file *m, void *v)

static void t_stop(struct seq_file *m, void *p)
{
+ mutex_unlock(&event_mutex);
}

static int
ftrace_event_seq_open(struct inode *inode, struct file *file)
{
- int ret;
const struct seq_operations *seq_ops;

if ((file->f_mode & FMODE_WRITE) &&
@@ -332,13 +340,7 @@ ftrace_event_seq_open(struct inode *inode, struct file *file)
ftrace_clear_events();

seq_ops = inode->i_private;
- ret = seq_open(file, seq_ops);
- if (!ret) {
- struct seq_file *m = file->private_data;
-
- m->private = ftrace_events.next;
- }
- return ret;
+ return seq_open(file, seq_ops);
}

static ssize_t
diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 97bd140..de1f821 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -433,6 +433,7 @@ static void filter_free_subsystem_preds(struct event_subsystem *system)
filter->n_preds = 0;
}

+ mutex_lock(&event_mutex);
list_for_each_entry(call, &ftrace_events, list) {
if (!call->define_fields)
continue;
@@ -442,6 +443,7 @@ static void filter_free_subsystem_preds(struct event_subsystem *system)
remove_filter_string(call->filter);
}
}
+ mutex_unlock(&event_mutex);
}

static int filter_add_pred_fn(struct filter_parse_state *ps,
@@ -622,6 +624,7 @@ static int filter_add_subsystem_pred(struct filter_parse_state *ps,
filter->preds[filter->n_preds] = pred;
filter->n_preds++;

+ mutex_lock(&event_mutex);
list_for_each_entry(call, &ftrace_events, list) {
int err;

@@ -633,12 +636,14 @@ static int filter_add_subsystem_pred(struct filter_parse_state *ps,

err = filter_add_pred(ps, call, pred);
if (err) {
+ mutex_unlock(&event_mutex);
filter_free_subsystem_preds(system);
parse_error(ps, FILT_ERR_BAD_SUBSYS_FILTER, 0);
return err;
}
replace_filter_string(call->filter, filter_string);
}
+ mutex_unlock(&event_mutex);

return 0;
}
--
1.5.4.rc3

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