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

PATCH Kay Sievers 2 participants 2 messages Apr 7, 2011 – Apr 12, 2011 Char and misc drivers

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From: Kay Sievers <[email protected]>

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 <[email protected]>
Signed-off-by Kay Sievers <[email protected]>
---
 mem.c |   41 ++++++++++++++++++++++++++---------------
 1 file changed, 26 insertions(+), 15 deletions(-)
drivers/char/mem.c +26 -15
index 436a990..78923a9 100644
@@ -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,
 };
#1 lore raw
Quoted 56 lines · On Thu, 07 Apr 2011 04:29:20 +0200 Kay Sievers <[email protected]> wrote:

From: Kay Sievers <[email protected]>

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().

...

drivers/char/mem.c +24 -14
@@ -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:

drivers/char/mem.c static const struct file_operations oldmem_fops = { · line 845

Quoted 10 lines
drivers/char/mem.c +2 -1
+	kfree(line);
 	return ret;
 }
 
 static const struct file_operations kmsg_fops = {
-	.write = kmsg_write,
+	.aio_write = kmsg_writev,
 	.llseek = noop_llseek,
 };

LGTM. Nitpicks:

From: Andrew Morton <[email protected]>

make `len' size_t, avoid multiple-assignments.

Cc Greg KH <[email protected]>
Cc Kay Sievers <[email protected]>
Cc Lennart Poettering <[email protected]>
Signed-off-by Andrew Morton <[email protected]>

---

 drivers/char/mem.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
drivers/char/mem.c renamed from drivers/char/mem.c~dev-kmsg-properly-support-writev-to-avoid-interleaved-printk-lines-fix +4 -3
diff -puN drivers/char/mem.c~dev-kmsg-properly-support-writev-to-avoid-interleaved-printk-lines-fix drivers/char/mem.c
@@ -810,11 +810,11 @@ static ssize_t kmsg_writev(struct kiocb 
 			   unsigned long count, loff_t pos)
 {
 	char *line, *p;
-	int len, i;
+	int i;
 	ssize_t ret = -EFAULT;
+	size_t len = iov_length(iv, count);
 
-	len = iov_length(iv, count);
-	line = p = kmalloc(len + 1, GFP_KERNEL);
+	line = kmalloc(len + 1, GFP_KERNEL);
 	if (line == NULL)
 		return -ENOMEM;
 

@@ -822,6 +822,7 @@ static ssize_t kmsg_writev(struct kiocb 
 	 * copy all vectors into a single string, to ensure we do
 	 * not interleave our log line with other printk calls
 	 */
+	p = line;
 	for (i = 0; i < count; i++) {
 		if (copy_from_user(p, iv[i].iov_base, iv[i].iov_len))
 			goto out;

_