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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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