[PATCH v7 6/7] printk: honor LOG_PREFIX in msg_print_text()

From: Alex Elder
Date: Mon Jul 28 2014 - 09:29:33 EST

This patch fixes a problem similar to what was addressed in the
previous patch.

All paths that read and format log records (for consoles, and for
reading via syslog and /dev/kmsg) go through msg_print_text(). That
function starts with some logic to determine whether the given log
record when formatted should begin with a "prefix" string, and
whether it should end with a newline. That logic has a bug.

The LOG_PREFIX flag in a log record indicates that when it's
formatted, a log record should include a prefix. This is used to
force a record to begin a new line--even if its preceding log record
contained LOG_CONT (indicating its content should be combined with
the next record).

Like the previous patch, the problem occurs when these flags are
all set:
prev & LOG_CONT
msg->flags & LOG_PREFIX
msg->flags & LOG_CONT
In that case, "prefix" should be true but it is not.

The fix involves checking LOG_PREFIX when a message has its LOG_CONT
flag set, and in that case allowing "prefix" to become false only
if LOG_PREFIX is not set. I.e., the logic for "prefix" would become:

if (prev & LOG_CONT && !(msg->flags & LOG_PREFIX))
prefix = false;
if (msg->flags & LOG_CONT)
if (prev & LOG_CONT && !(msg->flags & LOG_PREFIX))
prefix = false;

However, note that this makes the (msg->flags & LOG_CONT) block
redunant--it's handled by the test just above it. The result
becomes quite a bit simpler than before.

The following table concisely defines the problem:

prev | msg | msg ||
clear| clear| clear|| true
clear| clear| set || true
clear| set | clear|| true
clear| set | set || true
set | clear| clear|| false
set | clear| set || false
set | set | clear|| true
set | set | set || false <-- should be true

Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
Reviewed-by: Petr MlÃdek <pmladek@xxxxxxx>
kernel/printk/printk.c | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1eb07f4..982873b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1006,11 +1006,8 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
if ((prev & LOG_CONT) && !(msg->flags & LOG_PREFIX))
prefix = false;

- if (msg->flags & LOG_CONT) {
- if (prev & LOG_CONT)
- prefix = false;
+ if (msg->flags & LOG_CONT)
newline = false;
- }

if ((prev & LOG_CONT) && (msg->flags & LOG_PREFIX) && len < size) {
if (buf)

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