kmsg: /proc/kmsg - support reading of partial log records

Restore support for partial reads of any size on /proc/kmsg, in case the
supplied read buffer is smaller than the record size.

Some people seem to think is is ia good idea to run:
  $ dd if=/proc/kmsg bs=1 of=...
as a klog bridge.

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=44211
Reported-by: Jukka Ollila <jiiksteri@gmail.com>
Signed-off-by: Kay Sievers <kay@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/printk.c b/kernel/printk.c
index f02f1f5..50c3341 100644 (file)
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -217,6 +217,7 @@ static DEFINE_RAW_SPINLOCK(logbuf_lock);
 /* the next printk record to read by syslog(READ) or /proc/kmsg */
 static u64 syslog_seq;
 static u32 syslog_idx;
+static size_t syslog_partial;
 
 /* index and sequence number of the first record stored in the buffer */
 static u64 log_first_seq;
@@ -890,22 +891,33 @@ static int syslog_print(char __user *buf, int size)
 
        while (size > 0) {
                size_t n;
+               size_t skip;
 
                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;
+                       syslog_partial = 0;
                }
                if (syslog_seq == log_next_seq) {
                        raw_spin_unlock_irq(&logbuf_lock);
                        break;
                }
+
+               skip = syslog_partial;
                msg = log_from_idx(syslog_idx);
                n = msg_print_text(msg, true, text, LOG_LINE_MAX);
-               if (n <= size) {
+               if (n - syslog_partial <= size) {
+                       /* message fits into buffer, move forward */
                        syslog_idx = log_next(syslog_idx);
                        syslog_seq++;
+                       n -= syslog_partial;
+                       syslog_partial = 0;
+               } else if (!len){
+                       /* partial read(), remember position */
+                       n = size;
+                       syslog_partial += n;
                } else
                        n = 0;
                raw_spin_unlock_irq(&logbuf_lock);
@@ -913,17 +925,15 @@ static int syslog_print(char __user *buf, int size)
                if (!n)
                        break;
 
-               len += n;
-               size -= n;
-               buf += n;
-               n = copy_to_user(buf - n, text, n);
-
-               if (n) {
-                       len -= n;
+               if (copy_to_user(buf, text + skip, n)) {
                        if (!len)
                                len = -EFAULT;
                        break;
                }
+
+               len += n;
+               size -= n;
+               buf += n;
        }
 
        kfree(text);
@@ -1107,6 +1117,7 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
                        /* messages are gone, move to first one */
                        syslog_seq = log_first_seq;
                        syslog_idx = log_first_idx;
+                       syslog_partial = 0;
                }
                if (from_file) {
                        /*
@@ -1129,6 +1140,7 @@ int do_syslog(int type, char __user *buf, int len, bool from_file)
                                idx = log_next(idx);
                                seq++;
                        }
+                       error -= syslog_partial;
                }
                raw_spin_unlock_irq(&logbuf_lock);
                break;