[PATCH v7 5/7] printk: honor LOG_PREFIX in devkmsg_read()

From: Alex Elder
Date: Mon Jul 28 2014 - 09:30:12 EST

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"
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@xxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxx>
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 4b063c8..1eb07f4 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,

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