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>

1 files changed, 20 insertions, 8 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index f02f1f5ddc30..50c33411442d 100644
--- 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;