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    Subject[PATCH AUTOSEL 5.4 427/459] ftrace: fpid_next() should increase position index
    Date
    From: Vasily Averin <vvs@virtuozzo.com>

    [ Upstream commit e4075e8bdffd93a9b6d6e1d52fabedceeca5a91b ]

    if seq_file .next fuction does not change position index,
    read after some lseek can generate unexpected output.

    Without patch:
    # dd bs=4 skip=1 if=/sys/kernel/tracing/set_ftrace_pid
    dd: /sys/kernel/tracing/set_ftrace_pid: cannot skip to specified offset
    id
    no pid
    2+1 records in
    2+1 records out
    10 bytes copied, 0.000213285 s, 46.9 kB/s

    Notice the "id" followed by "no pid".

    With the patch:
    # dd bs=4 skip=1 if=/sys/kernel/tracing/set_ftrace_pid
    dd: /sys/kernel/tracing/set_ftrace_pid: cannot skip to specified offset
    id
    0+1 records in
    0+1 records out
    3 bytes copied, 0.000202112 s, 14.8 kB/s

    Notice that it only prints "id" and not the "no pid" afterward.

    Link: http://lkml.kernel.org/r/4f87c6ad-f114-30bb-8506-c32274ce2992@virtuozzo.com

    https://bugzilla.kernel.org/show_bug.cgi?id=206283
    Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/trace/ftrace.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
    index 407d8bf4ed93e..15160d707da45 100644
    --- a/kernel/trace/ftrace.c
    +++ b/kernel/trace/ftrace.c
    @@ -6537,9 +6537,10 @@ static void *fpid_next(struct seq_file *m, void *v, loff_t *pos)
    struct trace_array *tr = m->private;
    struct trace_pid_list *pid_list = rcu_dereference_sched(tr->function_pids);

    - if (v == FTRACE_NO_PIDS)
    + if (v == FTRACE_NO_PIDS) {
    + (*pos)++;
    return NULL;
    -
    + }
    return trace_pid_next(pid_list, v, pos);
    }

    --
    2.20.1
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