[PATCH v2 4/6] printk: honor LOG_PREFIX in msg_print_text()

[Alex Elder]

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  ||
     CONT |PREFIX| CONT ||prefix
    ------+------+------++------
     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>
---
 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 5d719bf..e5ee1cb 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;
-	}
 
 	do {
 		const char *next = memchr(text, '\n', text_size);
-- 
1.9.1

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