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    Subject[PATCH 1/7] tracing/events: don't increment @pos in s_start()
    While testing syscall tracepoints posted by Jason, I found 3 entries
    were missing when reading available_events. The output size of
    available_events is < 4 pages, which means we lost 1 entry per page.

    The cause is, it's wrong to increment @pos in s_start().

    Actually there's another bug here -- reading avaiable_events/set_events
    can race with module unload:

    # cat available_events |
    s_start() |
    s_stop() |
    | # rmmod foo.ko
    s_start() |
    call = list_entry(m->private) |

    @call might be freed and accessing it will lead to crash.

    [ Impact fix missing entries when reading available_events/set_events ]

    Reviewed-by: Liming Wang <liming.wang@windriver.com>
    Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
    ---
    kernel/trace/trace_events.c | 28 ++++++++++++++++++++++------
    1 files changed, 22 insertions(+), 6 deletions(-)

    diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
    index aa08be6..53c8fd3 100644
    --- a/kernel/trace/trace_events.c
    +++ b/kernel/trace/trace_events.c
    @@ -300,10 +300,18 @@ t_next(struct seq_file *m, void *v, loff_t *pos)

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

    static void *
    @@ -332,10 +340,18 @@ s_next(struct seq_file *m, void *v, loff_t *pos)

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

    static int t_show(struct seq_file *m, void *v)
    --
    1.5.4.rc3

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