printk: /dev/kmsg - properly support writev() to avoid interleaved printk() lines

printk: /dev/kmsg - properly support writev() to avoid interleaved printk lines

We should avoid calling printk() in a loop, when we pass a single
string to /dev/kmsg with writev().

Cc: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 26 insertions, 15 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 436a9901799..78923a9f534 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -806,29 +806,40 @@ static const struct file_operations oldmem_fops = {
 };
 #endif
 
-static ssize_t kmsg_write(struct file *file, const char __user *buf,
-                          size_t count, loff_t *ppos)
+static ssize_t kmsg_writev(struct kiocb *iocb, const struct iovec *iv,
+                           unsigned long count, loff_t pos)
 {
-        char *tmp;
-        ssize_t ret;
+        char *line, *p;
+        int len, i;
+        ssize_t ret = -EFAULT;
 
-        tmp = kmalloc(count + 1, GFP_KERNEL);
-        if (tmp == NULL)
+        len = iov_length(iv, count);
+        line = p = kmalloc(len + 1, GFP_KERNEL);
+        if (line == NULL)
                 return -ENOMEM;
-        ret = -EFAULT;
-        if (!copy_from_user(tmp, buf, count)) {
-                tmp[count] = 0;
-                ret = printk("%s", tmp);
-                if (ret > count)
-                        /* printk can add a prefix */
-                        ret = count;
+
+        /*
+         * copy all vectors into a single string, to ensure we do
+         * not interleave our log line with other printk calls
+         */
+        for (i = 0; i < count; i++) {
+                if (copy_from_user(p, iv[i].iov_base, iv[i].iov_len))
+                        goto out;
+                p += iv[i].iov_len;
         }
-        kfree(tmp);
+        p[0] = '\0';
+
+        ret = printk("%s", line);
+        /* printk can add a prefix */
+        if (ret > len)
+                ret = len;
+out:
+        kfree(line);
         return ret;
 }
 
 static const struct file_operations kmsg_fops = {
-        .write = kmsg_write,
+        .aio_write = kmsg_writev,
         .llseek = noop_llseek,
 };