2011-04-07 02:30:08

by Kay Sievers

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

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(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 436a990..78923a9 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,
};

2011-04-12 23:13:49

by Andrew Morton

Subject: Re: printk: /dev/kmsg - properly support writev() to avoid interleaved printk() 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().
>
> ...
> --- 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,
> };
>

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(-)

diff -puN drivers/char/mem.c~dev-kmsg-properly-support-writev-to-avoid-interleaved-printk-lines-fix drivers/char/mem.c
--- a/drivers/char/mem.c~dev-kmsg-properly-support-writev-to-avoid-interleaved-printk-lines-fix
+++ a/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;
_