2 files changed, 41 insertions, 11 deletions

diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
index 281ecc5f9709..7e7e07a82e0e 100644
--- a/Documentation/ABI/testing/dev-kmsg
+++ b/Documentation/ABI/testing/dev-kmsg
@@ -58,16 +58,18 @@ Description:	The /dev/kmsg character device node provides userspace access
 
                 The output format consists of a prefix carrying the syslog
                 prefix including priority and facility, the 64 bit message
-                sequence number and the monotonic timestamp in microseconds.
-                The values are separated by a ','. Future extensions might
-                add more comma separated values before the terminating ';'.
-                Unknown values should be gracefully ignored.
+                sequence number and the monotonic timestamp in microseconds,
+                and a flag field. All fields are separated by a ','.
+
+                Future extensions might add more comma separated values before
+                the terminating ';'. Unknown fields and values should be
+                gracefully ignored.
 
                 The human readable text string starts directly after the ';'
                 and is terminated by a '\n'. Untrusted values derived from
                 hardware or other facilities are printed, therefore
-                all non-printable characters in the log message are escaped
-                by "\x00" C-style hex encoding.
+                all non-printable characters and '\' itself in the log message
+                are escaped by "\x00" C-style hex encoding.
 
                 A line starting with ' ', is a continuation line, adding
                 key/value pairs to the log message, which provide the machine
@@ -75,11 +77,11 @@ Description:	The /dev/kmsg character device node provides userspace access
                 userspace.
 
                 Example:
-                7,160,424069;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)
+                7,160,424069,-;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)
                  SUBSYSTEM=acpi
                  DEVICE=+acpi:PNP0A03:00
-                6,339,5140900;NET: Registered protocol family 10
-                30,340,5690716;udevd[80]: starting version 181
+                6,339,5140900,-;NET: Registered protocol family 10
+                30,340,5690716,-;udevd[80]: starting version 181
 
                 The DEVICE= key uniquely identifies devices the following way:
                   b12:8        - block dev_t
@@ -87,4 +89,13 @@ Description:	The /dev/kmsg character device node provides userspace access
                   n8           - netdev ifindex
                   +sound:card0 - subsystem:devname
 
+                The flags field carries '-' by default. A 'c' indicates a
+                fragment of a line. All following fragments are flagged with
+                '+'. Note, that these hints about continuation lines are not
+                neccessarily correct, and the stream could be interleaved with
+                unrelated messages, but merging the lines in the output
+                usually produces better human readable results. A similar
+                logic is used internally when messages are printed to the
+                console, /proc/kmsg or the syslog() syscall.
+
 Users:          dmesg(1), userspace kernel log consumers
diff --git a/kernel/printk.c b/kernel/printk.c
index 6c3d5bf14da2..a41106e19077 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -361,6 +361,7 @@ static void log_store(int facility, int level,
 struct devkmsg_user {
         u64 seq;
         u32 idx;
+        enum log_flags prev;
         struct mutex lock;
         char buf[8192];
 };
@@ -426,6 +427,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
         struct log *msg;
         u64 ts_usec;
         size_t i;
+        char cont = '-';
         size_t len;
         ssize_t ret;
 
@@ -463,8 +465,25 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
         msg = log_from_idx(user->idx);
         ts_usec = msg->ts_nsec;
         do_div(ts_usec, 1000);
-        len = sprintf(user->buf, "%u,%llu,%llu;",
-                      (msg->facility << 3) | msg->level, user->seq, ts_usec);
+
+        /*
+         * If we couldn't merge continuation line fragments during the print,
+         * export the stored flags to allow an optional external merge of the
+         * records. Merging the records isn't always neccessarily correct, like
+         * when we hit a race during printing. In most cases though, it produces
+         * 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)))
+                cont = '+';
+
+        len = sprintf(user->buf, "%u,%llu,%llu,%c;",
+                      (msg->facility << 3) | msg->level,
+                      user->seq, ts_usec, cont);
+        user->prev = msg->flags;
 
         /* escape non-printable characters */
         for (i = 0; i < msg->text_len; i++) {