1 files changed, 418 insertions, 114 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 32462d2b364a..dba18211685e 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -193,12 +193,19 @@ static int console_may_schedule;
  * separated by ',', and find the message after the ';' character.
  */
 
+enum log_flags {
+        LOG_DEFAULT = 0,
+        LOG_NOCONS = 1,         /* already flushed, do not print to console */
+};
+
 struct log {
         u64 ts_nsec;            /* timestamp in nanoseconds */
         u16 len;                /* length of entire record */
         u16 text_len;           /* length of text buffer */
         u16 dict_len;           /* length of dictionary buffer */
-        u16 level;              /* syslog level + facility */
+        u8 facility;            /* syslog facility */
+        u8 flags:5;             /* internal record flags */
+        u8 level:3;             /* syslog level */
 };
 
 /*
@@ -227,10 +234,10 @@ static u32 clear_idx;
 #define LOG_LINE_MAX 1024
 
 /* record buffer */
-#if !defined(CONFIG_64BIT) || defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
+#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
 #define LOG_ALIGN 4
 #else
-#define LOG_ALIGN 8
+#define LOG_ALIGN __alignof__(struct log)
 #endif
 #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
 static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
@@ -286,6 +293,7 @@ static u32 log_next(u32 idx)
 
 /* insert record into the buffer, discard old ones, update heads */
 static void log_store(int facility, int level,
+                      enum log_flags flags, u64 ts_nsec,
                       const char *dict, u16 dict_len,
                       const char *text, u16 text_len)
 {
@@ -329,8 +337,13 @@ static void log_store(int facility, int level,
         msg->text_len = text_len;
         memcpy(log_dict(msg), dict, dict_len);
         msg->dict_len = dict_len;
-        msg->level = (facility << 3) | (level & 7);
-        msg->ts_nsec = local_clock();
+        msg->facility = facility;
+        msg->level = level & 7;
+        msg->flags = flags & 0x1f;
+        if (ts_nsec > 0)
+                msg->ts_nsec = ts_nsec;
+        else
+                msg->ts_nsec = local_clock();
         memset(log_dict(msg) + dict_len, 0, pad_len);
         msg->len = sizeof(struct log) + text_len + dict_len + pad_len;
 
@@ -414,7 +427,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
         if (!user)
                 return -EBADF;
 
-        mutex_lock(&user->lock);
+        ret = mutex_lock_interruptible(&user->lock);
+        if (ret)
+                return ret;
         raw_spin_lock(&logbuf_lock);
         while (user->seq == log_next_seq) {
                 if (file->f_flags & O_NONBLOCK) {
@@ -444,7 +459,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
         ts_usec = msg->ts_nsec;
         do_div(ts_usec, 1000);
         len = sprintf(user->buf, "%u,%llu,%llu;",
-                      msg->level, user->seq, ts_usec);
+                      (msg->facility << 3) | msg->level, user->seq, ts_usec);
 
         /* escape non-printable characters */
         for (i = 0; i < msg->text_len; i++) {
@@ -785,6 +800,21 @@ static bool printk_time;
 #endif
 module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
 
+static size_t print_time(u64 ts, char *buf)
+{
+        unsigned long rem_nsec;
+
+        if (!printk_time)
+                return 0;
+
+        if (!buf)
+                return 15;
+
+        rem_nsec = do_div(ts, 1000000000);
+        return sprintf(buf, "[%5lu.%06lu] ",
+                       (unsigned long)ts, rem_nsec / 1000);
+}
+
 static size_t print_prefix(const struct log *msg, bool syslog, char *buf)
 {
         size_t len = 0;
@@ -801,18 +831,7 @@ static size_t print_prefix(const struct log *msg, bool syslog, char *buf)
                 }
         }
 
-        if (printk_time) {
-                if (buf) {
-                        unsigned long long ts = msg->ts_nsec;
-                        unsigned long rem_nsec = do_div(ts, 1000000000);
-
-                        len += sprintf(buf + len, "[%5lu.%06lu] ",
-                                         (unsigned long) ts, rem_nsec / 1000);
-                } else {
-                        len += 15;
-                }
-        }
-
+        len += print_time(msg->ts_nsec, buf ? buf + len : NULL);
         return len;
 }
 
@@ -860,26 +879,49 @@ static int syslog_print(char __user *buf, int size)
 {
         char *text;
         struct log *msg;
-        int len;
+        int len = 0;
 
         text = kmalloc(LOG_LINE_MAX, GFP_KERNEL);
         if (!text)
                 return -ENOMEM;
 
-        raw_spin_lock_irq(&logbuf_lock);
-        if (syslog_seq < log_first_seq) {
-                /* messages are gone, move to first one */
-                syslog_seq = log_first_seq;
-                syslog_idx = log_first_idx;
-        }
-        msg = log_from_idx(syslog_idx);
-        len = msg_print_text(msg, true, text, LOG_LINE_MAX);
-        syslog_idx = log_next(syslog_idx);
-        syslog_seq++;
-        raw_spin_unlock_irq(&logbuf_lock);
+        while (size > 0) {
+                size_t n;
+
+                raw_spin_lock_irq(&logbuf_lock);
+                if (syslog_seq < log_first_seq) {
+                        /* messages are gone, move to first one */
+                        syslog_seq = log_first_seq;
+                        syslog_idx = log_first_idx;
+                }
+                if (syslog_seq == log_next_seq) {
+                        raw_spin_unlock_irq(&logbuf_lock);
+                        break;
+                }
+                msg = log_from_idx(syslog_idx);
+                n = msg_print_text(msg, true, text, LOG_LINE_MAX);
+                if (n <= size) {
+                        syslog_idx = log_next(syslog_idx);
+                        syslog_seq++;
+                } else
+                        n = 0;
+                raw_spin_unlock_irq(&logbuf_lock);
+
+                if (!n)
+                        break;
+
+                len += n;
+                size -= n;
+                buf += n;
+                n = copy_to_user(buf - n, text, n);
 
-        if (len > 0 && copy_to_user(buf, text, len))
-                len = -EFAULT;
+                if (n) {
+                        len -= n;
+                        if (!len)
+                                len = -EFAULT;
+                        break;
+                }
+        }
 
         kfree(text);
         return len;
@@ -909,7 +951,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
                 /*
                  * Find first record that fits, including all following records,
                  * into the user-provided buffer for this dump.
-                */
+                 */
                 seq = clear_seq;
                 idx = clear_idx;
                 while (seq < log_next_seq) {
@@ -919,6 +961,8 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
                         idx = log_next(idx);
                         seq++;
                 }
+
+                /* move first record forward until length fits into the buffer */
                 seq = clear_seq;
                 idx = clear_idx;
                 while (len > size && seq < log_next_seq) {
@@ -929,7 +973,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
                         seq++;
                 }
 
-                /* last message in this dump */
+                /* last message fitting into this dump */
                 next_seq = log_next_seq;
 
                 len = 0;
@@ -974,6 +1018,7 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
 {
         bool clear = false;
         static int saved_console_loglevel = -1;
+        static DEFINE_MUTEX(syslog_mutex);
         int error;
 
         error = check_syslog_permissions(type, from_file);
@@ -1000,11 +1045,17 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
                         error = -EFAULT;
                         goto out;
                 }
+                error = mutex_lock_interruptible(&syslog_mutex);
+                if (error)
+                        goto out;
                 error = wait_event_interruptible(log_wait,
                                                  syslog_seq != log_next_seq);
-                if (error)
+                if (error) {
+                        mutex_unlock(&syslog_mutex);
                         goto out;
+                }
                 error = syslog_print(buf, len);
+                mutex_unlock(&syslog_mutex);
                 break;
         /* Read/clear last kernel messages */
         case SYSLOG_ACTION_READ_CLEAR:
@@ -1027,6 +1078,7 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
         /* Clear ring buffer */
         case SYSLOG_ACTION_CLEAR:
                 syslog_print_all(NULL, 0, true);
+                break;
         /* Disable logging to console */
         case SYSLOG_ACTION_CONSOLE_OFF:
                 if (saved_console_loglevel == -1)
@@ -1259,15 +1311,92 @@ static inline void printk_delay(void)
         }
 }
 
+/*
+ * Continuation lines are buffered, and not committed to the record buffer
+ * until the line is complete, or a race forces it. The line fragments
+ * though, are printed immediately to the consoles to ensure everything has
+ * reached the console in case of a kernel crash.
+ */
+static struct cont {
+        char buf[LOG_LINE_MAX];
+        size_t len;                     /* length == 0 means unused buffer */
+        size_t cons;                    /* bytes written to console */
+        struct task_struct *owner;      /* task of first print*/
+        u64 ts_nsec;                    /* time of first print */
+        u8 level;                       /* log level of first message */
+        u8 facility;                    /* log level of first message */
+        bool flushed:1;                 /* buffer sealed and committed */
+} cont;
+
+static void cont_flush(void)
+{
+        if (cont.flushed)
+                return;
+        if (cont.len == 0)
+                return;
+
+        log_store(cont.facility, cont.level, LOG_NOCONS, cont.ts_nsec,
+                  NULL, 0, cont.buf, cont.len);
+
+        cont.flushed = true;
+}
+
+static bool cont_add(int facility, int level, const char *text, size_t len)
+{
+        if (cont.len && cont.flushed)
+                return false;
+
+        if (cont.len + len > sizeof(cont.buf)) {
+                cont_flush();
+                return false;
+        }
+
+        if (!cont.len) {
+                cont.facility = facility;
+                cont.level = level;
+                cont.owner = current;
+                cont.ts_nsec = local_clock();
+                cont.cons = 0;
+                cont.flushed = false;
+        }
+
+        memcpy(cont.buf + cont.len, text, len);
+        cont.len += len;
+        return true;
+}
+
+static size_t cont_print_text(char *text, size_t size)
+{
+        size_t textlen = 0;
+        size_t len;
+
+        if (cont.cons == 0) {
+                textlen += print_time(cont.ts_nsec, text);
+                size -= textlen;
+        }
+
+        len = cont.len - cont.cons;
+        if (len > 0) {
+                if (len+1 > size)
+                        len = size-1;
+                memcpy(text + textlen, cont.buf + cont.cons, len);
+                textlen += len;
+                cont.cons = cont.len;
+        }
+
+        if (cont.flushed) {
+                text[textlen++] = '\n';
+                /* got everything, release buffer */
+                cont.len = 0;
+        }
+        return textlen;
+}
+
 asmlinkage int vprintk_emit(int facility, int level,
                             const char *dict, size_t dictlen,
                             const char *fmt, va_list args)
 {
         static int recursion_bug;
-        static char cont_buf[LOG_LINE_MAX];
-        static size_t cont_len;
-        static int cont_level;
-        static struct task_struct *cont_task;
         static char textbuf[LOG_LINE_MAX];
         char *text = textbuf;
         size_t text_len;
@@ -1313,7 +1442,8 @@ asmlinkage int vprintk_emit(int facility, int level,
                 recursion_bug = 0;
                 printed_len += strlen(recursion_msg);
                 /* emit KERN_CRIT message */
-                log_store(0, 2, NULL, 0, recursion_msg, printed_len);
+                log_store(0, 2, LOG_DEFAULT, 0,
+                          NULL, 0, recursion_msg, printed_len);
         }
 
         /*
@@ -1351,55 +1481,37 @@ asmlinkage int vprintk_emit(int facility, int level,
         }
 
         if (!newline) {
-                if (cont_len && (prefix || cont_task != current)) {
-                        /*
-                         * Flush earlier buffer, which is either from a
-                         * different thread, or when we got a new prefix.
-                         */
-                        log_store(facility, cont_level, NULL, 0, cont_buf, cont_len);
-                        cont_len = 0;
-                }
-
-                if (!cont_len) {
-                        cont_level = level;
-                        cont_task = current;
-                }
+                /*
+                 * Flush the conflicting buffer. An earlier newline was missing,
+                 * or another task also prints continuation lines.
+                 */
+                if (cont.len && (prefix || cont.owner != current))
+                        cont_flush();
 
-                /* buffer or append to earlier buffer from the same thread */
-                if (cont_len + text_len > sizeof(cont_buf))
-                        text_len = sizeof(cont_buf) - cont_len;
-                memcpy(cont_buf + cont_len, text, text_len);
-                cont_len += text_len;
+                /* buffer line if possible, otherwise store it right away */
+                if (!cont_add(facility, level, text, text_len))
+                        log_store(facility, level, LOG_DEFAULT, 0,
+                                  dict, dictlen, text, text_len);
         } else {
-                if (cont_len && cont_task == current) {
-                        if (prefix) {
-                                /*
-                                 * New prefix from the same thread; flush. We
-                                 * either got no earlier newline, or we race
-                                 * with an interrupt.
-                                 */
-                                log_store(facility, cont_level,
-                                          NULL, 0, cont_buf, cont_len);
-                                cont_len = 0;
-                        }
+                bool stored = false;
 
-                        /* append to the earlier buffer and flush */
-                        if (cont_len + text_len > sizeof(cont_buf))
-                                text_len = sizeof(cont_buf) - cont_len;
-                        memcpy(cont_buf + cont_len, text, text_len);
-                        cont_len += text_len;
-                        log_store(facility, cont_level,
-                                  NULL, 0, cont_buf, cont_len);
-                        cont_len = 0;
-                        cont_task = NULL;
-                        printed_len = cont_len;
-                } else {
-                        /* ordinary single and terminated line */
-                        log_store(facility, level,
-                                  dict, dictlen, text, text_len);
-                        printed_len = text_len;
+                /*
+                 * If an earlier newline was missing and it was the same task,
+                 * either merge it with the current buffer and flush, or if
+                 * there was a race with interrupts (prefix == true) then just
+                 * flush it out and store this line separately.
+                 */
+                if (cont.len && cont.owner == current) {
+                        if (!prefix)
+                                stored = cont_add(facility, level, text, text_len);
+                        cont_flush();
                 }
+
+                if (!stored)
+                        log_store(facility, level, LOG_DEFAULT, 0,
+                                  dict, dictlen, text, text_len);
         }
+        printed_len += text_len;
 
         /*
          * Try to acquire and then immediately release the console semaphore.
@@ -1486,11 +1598,18 @@ EXPORT_SYMBOL(printk);
 #else
 
 #define LOG_LINE_MAX 0
+static struct cont {
+        size_t len;
+        size_t cons;
+        u8 level;
+        bool flushed:1;
+} cont;
 static struct log *log_from_idx(u32 idx) { return NULL; }
 static u32 log_next(u32 idx) { return 0; }
 static void call_console_drivers(int level, const char *text, size_t len) {}
 static size_t msg_print_text(const struct log *msg, bool syslog,
                              char *buf, size_t size) { return 0; }
+static size_t cont_print_text(char *text, size_t size) { return 0; }
 
 #endif /* CONFIG_PRINTK */
 
@@ -1782,6 +1901,7 @@ static u32 console_idx;
  */
 void console_unlock(void)
 {
+        static char text[LOG_LINE_MAX];
         static u64 seen_seq;
         unsigned long flags;
         bool wake_klogd = false;
@@ -1794,10 +1914,23 @@ void console_unlock(void)
 
         console_may_schedule = 0;
 
+        /* flush buffered message fragment immediately to console */
+        raw_spin_lock_irqsave(&logbuf_lock, flags);
+        if (cont.len && (cont.cons < cont.len || cont.flushed)) {
+                size_t len;
+
+                len = cont_print_text(text, sizeof(text));
+                raw_spin_unlock(&logbuf_lock);
+                stop_critical_timings();
+                call_console_drivers(cont.level, text, len);
+                start_critical_timings();
+                local_irq_restore(flags);
+        } else
+                raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+
 again:
         for (;;) {
                 struct log *msg;
-                static char text[LOG_LINE_MAX];
                 size_t len;
                 int level;
 
@@ -1812,13 +1945,22 @@ again:
                         console_seq = log_first_seq;
                         console_idx = log_first_idx;
                 }
-
+skip:
                 if (console_seq == log_next_seq)
                         break;
 
                 msg = log_from_idx(console_idx);
-                level = msg->level & 7;
+                if (msg->flags & LOG_NOCONS) {
+                        /*
+                         * Skip record we have buffered and already printed
+                         * directly to the console when we received it.
+                         */
+                        console_idx = log_next(console_idx);
+                        console_seq++;
+                        goto skip;
+                }
 
+                level = msg->level;
                 len = msg_print_text(msg, false, text, sizeof(text));
 
                 console_idx = log_next(console_idx);
@@ -2300,48 +2442,210 @@ module_param_named(always_kmsg_dump, always_kmsg_dump, bool, S_IRUGO | S_IWUSR);
  * kmsg_dump - dump kernel log to kernel message dumpers.
  * @reason: the reason (oops, panic etc) for dumping
  *
- * Iterate through each of the dump devices and call the oops/panic
- * callbacks with the log buffer.
+ * Call each of the registered dumper's dump() callback, which can
+ * retrieve the kmsg records with kmsg_dump_get_line() or
+ * kmsg_dump_get_buffer().
  */
 void kmsg_dump(enum kmsg_dump_reason reason)
 {
-        u64 idx;
         struct kmsg_dumper *dumper;
-        const char *s1, *s2;
-        unsigned long l1, l2;
         unsigned long flags;
 
         if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)
                 return;
 
-        /* Theoretically, the log could move on after we do this, but
-           there's not a lot we can do about that. The new messages
-           will overwrite the start of what we dump. */
+        rcu_read_lock();
+        list_for_each_entry_rcu(dumper, &dump_list, list) {
+                if (dumper->max_reason && reason > dumper->max_reason)
+                        continue;
+
+                /* initialize iterator with data about the stored records */
+                dumper->active = true;
+
+                raw_spin_lock_irqsave(&logbuf_lock, flags);
+                dumper->cur_seq = clear_seq;
+                dumper->cur_idx = clear_idx;
+                dumper->next_seq = log_next_seq;
+                dumper->next_idx = log_next_idx;
+                raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+
+                /* invoke dumper which will iterate over records */
+                dumper->dump(dumper, reason);
+
+                /* reset iterator */
+                dumper->active = false;
+        }
+        rcu_read_unlock();
+}
+
+/**
+ * kmsg_dump_get_line - retrieve one kmsg log line
+ * @dumper: registered kmsg dumper
+ * @syslog: include the "<4>" prefixes
+ * @line: buffer to copy the line to
+ * @size: maximum size of the buffer
+ * @len: length of line placed into buffer
+ *
+ * Start at the beginning of the kmsg buffer, with the oldest kmsg
+ * record, and copy one record into the provided buffer.
+ *
+ * Consecutive calls will return the next available record moving
+ * towards the end of the buffer with the youngest messages.
+ *
+ * A return value of FALSE indicates that there are no more records to
+ * read.
+ */
+bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
+                        char *line, size_t size, size_t *len)
+{
+        unsigned long flags;
+        struct log *msg;
+        size_t l = 0;
+        bool ret = false;
+
+        if (!dumper->active)
+                goto out;
 
         raw_spin_lock_irqsave(&logbuf_lock, flags);
-        if (syslog_seq < log_first_seq)
-                idx = syslog_idx;
-        else
-                idx = log_first_idx;
+        if (dumper->cur_seq < log_first_seq) {
+                /* messages are gone, move to first available one */
+                dumper->cur_seq = log_first_seq;
+                dumper->cur_idx = log_first_idx;
+        }
 
-        if (idx > log_next_idx) {
-                s1 = log_buf;
-                l1 = log_next_idx;
+        /* last entry */
+        if (dumper->cur_seq >= log_next_seq) {
+                raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+                goto out;
+        }
 
-                s2 = log_buf + idx;
-                l2 = log_buf_len - idx;
-        } else {
-                s1 = "";
-                l1 = 0;
+        msg = log_from_idx(dumper->cur_idx);
+        l = msg_print_text(msg, syslog,
+                              line, size);
 
-                s2 = log_buf + idx;
-                l2 = log_next_idx - idx;
+        dumper->cur_idx = log_next(dumper->cur_idx);
+        dumper->cur_seq++;
+        ret = true;
+        raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+out:
+        if (len)
+                *len = l;
+        return ret;
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_get_line);
+
+/**
+ * kmsg_dump_get_buffer - copy kmsg log lines
+ * @dumper: registered kmsg dumper
+ * @syslog: include the "<4>" prefixes
+ * @buf: buffer to copy the line to
+ * @size: maximum size of the buffer
+ * @len: length of line placed into buffer
+ *
+ * Start at the end of the kmsg buffer and fill the provided buffer
+ * with as many of the the *youngest* kmsg records that fit into it.
+ * If the buffer is large enough, all available kmsg records will be
+ * copied with a single call.
+ *
+ * Consecutive calls will fill the buffer with the next block of
+ * available older records, not including the earlier retrieved ones.
+ *
+ * A return value of FALSE indicates that there are no more records to
+ * read.
+ */
+bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
+                          char *buf, size_t size, size_t *len)
+{
+        unsigned long flags;
+        u64 seq;
+        u32 idx;
+        u64 next_seq;
+        u32 next_idx;
+        size_t l = 0;
+        bool ret = false;
+
+        if (!dumper->active)
+                goto out;
+
+        raw_spin_lock_irqsave(&logbuf_lock, flags);
+        if (dumper->cur_seq < log_first_seq) {
+                /* messages are gone, move to first available one */
+                dumper->cur_seq = log_first_seq;
+                dumper->cur_idx = log_first_idx;
         }
+
+        /* last entry */
+        if (dumper->cur_seq >= dumper->next_seq) {
+                raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+                goto out;
+        }
+
+        /* calculate length of entire buffer */
+        seq = dumper->cur_seq;
+        idx = dumper->cur_idx;
+        while (seq < dumper->next_seq) {
+                struct log *msg = log_from_idx(idx);
+
+                l += msg_print_text(msg, true, NULL, 0);
+                idx = log_next(idx);
+                seq++;
+        }
+
+        /* move first record forward until length fits into the buffer */
+        seq = dumper->cur_seq;
+        idx = dumper->cur_idx;
+        while (l > size && seq < dumper->next_seq) {
+                struct log *msg = log_from_idx(idx);
+
+                l -= msg_print_text(msg, true, NULL, 0);
+                idx = log_next(idx);
+                seq++;
+        }
+
+        /* last message in next interation */
+        next_seq = seq;
+        next_idx = idx;
+
+        l = 0;
+        while (seq < dumper->next_seq) {
+                struct log *msg = log_from_idx(idx);
+
+                l += msg_print_text(msg, syslog,
+                                    buf + l, size - l);
+
+                idx = log_next(idx);
+                seq++;
+        }
+
+        dumper->next_seq = next_seq;
+        dumper->next_idx = next_idx;
+        ret = true;
         raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+out:
+        if (len)
+                *len = l;
+        return ret;
+}
+EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
 
-        rcu_read_lock();
-        list_for_each_entry_rcu(dumper, &dump_list, list)
-                dumper->dump(dumper, reason, s1, l1, s2, l2);
-        rcu_read_unlock();
+/**
+ * kmsg_dump_rewind - reset the interator
+ * @dumper: registered kmsg dumper
+ *
+ * Reset the dumper's iterator so that kmsg_dump_get_line() and
+ * kmsg_dump_get_buffer() can be called again and used multiple
+ * times within the same dumper.dump() callback.
+ */
+void kmsg_dump_rewind(struct kmsg_dumper *dumper)
+{
+        unsigned long flags;
+
+        raw_spin_lock_irqsave(&logbuf_lock, flags);
+        dumper->cur_seq = clear_seq;
+        dumper->cur_idx = clear_idx;
+        dumper->next_seq = log_next_seq;
+        dumper->next_idx = log_next_idx;
+        raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 }
+EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 #endif