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    Subject[for-next][PATCH 07/23] tracing: Fix return value of trace_pid_write()
    From: Wonhyuk Yang <vvghjk1234@gmail.com>

    Setting set_event_pid with trailing whitespace lead to endless write
    system calls like below.

    $ strace echo "123 " > /sys/kernel/debug/tracing/set_event_pid
    execve("/usr/bin/echo", ["echo", "123 "], ...) = 0
    ...
    write(1, "123 \n", 5) = 4
    write(1, "\n", 1) = 0
    write(1, "\n", 1) = 0
    write(1, "\n", 1) = 0
    write(1, "\n", 1) = 0
    write(1, "\n", 1) = 0
    ....

    This is because, the result of trace_get_user's are not returned when it
    read at least one pid. To fix it, update read variable even if
    parser->idx == 0.

    The result of applied patch is below.

    $ strace echo "123 " > /sys/kernel/debug/tracing/set_event_pid
    execve("/usr/bin/echo", ["echo", "123 "], ...) = 0
    ...
    write(1, "123 \n", 5) = 5
    close(1) = 0

    Link: https://lkml.kernel.org/r/20220503050546.288911-1-vvghjk1234@gmail.com

    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Baik Song An <bsahn@etri.re.kr>
    Cc: Hong Yeon Kim <kimhy@etri.re.kr>
    Cc: Taeung Song <taeung@reallinux.co.kr>
    Cc: linuxgeek@linuxgeek.io
    Cc: stable@vger.kernel.org
    Fixes: 4909010788640 ("tracing: Add set_event_pid directory for future use")
    Signed-off-by: Wonhyuk Yang <vvghjk1234@gmail.com>
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
    ---
    kernel/trace/trace.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
    index 498ae22d4ffa..4825883b2ffd 100644
    --- a/kernel/trace/trace.c
    +++ b/kernel/trace/trace.c
    @@ -721,13 +721,16 @@ int trace_pid_write(struct trace_pid_list *filtered_pids,
    pos = 0;

    ret = trace_get_user(&parser, ubuf, cnt, &pos);
    - if (ret < 0 || !trace_parser_loaded(&parser))
    + if (ret < 0)
    break;

    read += ret;
    ubuf += ret;
    cnt -= ret;

    + if (!trace_parser_loaded(&parser))
    + break;
    +
    ret = -EINVAL;
    if (kstrtoul(parser.buffer, 0, &val))
    break;
    @@ -753,7 +756,6 @@ int trace_pid_write(struct trace_pid_list *filtered_pids,
    if (!nr_pids) {
    /* Cleared the list of pids */
    trace_pid_list_free(pid_list);
    - read = ret;
    pid_list = NULL;
    }

    --
    2.35.1
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