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    Subject[PATCH v2 3/6] printk: honor LOG_PREFIX in devkmsg_read()
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    In devkmsg_read(), a variable "cont" holds a character that's used
    to indicate whether a given log line is a "continuation", that is,
    whether a log record should be merged with the one before or after
    it. If a record should be merged with its successor (but not its
    predecessor) that character is 'c'. And the line following such a
    'c' log record is normally marked with a '+' to show it is continues
    its predecessor. Any other cases are marked '-', indicating the
    log record stands on its own.

    There is an exception. If a log record is marked LOG_PREFIX, it
    indicates that this record represents a new log entry, implicitly
    terminating the predecessor--even if the predecessor would otherwise
    have been continued. So a record marked LOG_PREFIX (that is not
    also marked LOG_CONT) should have '-' for its "cont" variable.

    The logic that determines this "continuation" character has a bug
    that gets that exceptional case wrong.

    The specific case that produces the wrong result is when all of
    these conditions are non-zero:
    user->prev & LOG_CONT
    msg->flags & LOG_PREFIX
    msg->flags & LOG_CONT
    The bug is that despite the message's LOG_PREFIX flag, the
    "cont" character is getting set to '+' rather than 'c'.

    The problem is that the message's LOG_PREFIX flag is getting
    ignored if its LOG_CONT flag is also set. Rearrange the logic
    here to produce the correct result.

    The following table concisely defines the problem:

    prev | msg | msg ||"cont"
    CONT |PREFIX| CONT || char
    ------+------+------++------
    clear| clear| clear|| '-'
    clear| clear| set || 'c'
    clear| set | clear|| '-'
    clear| set | set || 'c'
    set | clear| clear|| '+'
    set | clear| set || '+'
    set | set | clear|| '-'
    set | set | set || '+' <-- should be 'c'

    Signed-off-by: Alex Elder <elder@linaro.org>
    Reviewed-by: Petr Mladek <pmladek@suse.cz>
    ---
    kernel/printk/printk.c | 11 ++++++-----
    1 file changed, 6 insertions(+), 5 deletions(-)

    diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    index 1ee1862..5d719bf 100644
    --- a/kernel/printk/printk.c
    +++ b/kernel/printk/printk.c
    @@ -572,7 +572,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
    struct printk_log *msg;
    u64 ts_usec;
    size_t i;
    - char cont = '-';
    + char cont;
    size_t len;
    ssize_t ret;

    @@ -619,11 +619,12 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
    * better readable output. 'c' in the record flags mark the first
    * fragment of a line, '+' the following.
    */
    - if (msg->flags & LOG_CONT && !(user->prev & LOG_CONT))
    - cont = 'c';
    - else if ((msg->flags & LOG_CONT) ||
    - ((user->prev & LOG_CONT) && !(msg->flags & LOG_PREFIX)))
    + if ((user->prev & LOG_CONT) && !(msg->flags & LOG_PREFIX))
    cont = '+';
    + else if (msg->flags & LOG_CONT)
    + cont = 'c';
    + else
    + cont = '-';

    len = sprintf(user->buf, "%u,%llu,%llu,%c;",
    (msg->facility << 3) | msg->level,
    --
    1.9.1


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