linux-kernel - Re: [PATCH 3/4] HID: debug: empty output queue on read

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Date:	Thu, 18 Apr 2013 18:55:47 -0700 (PDT)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Benjamin Tissoires <benjamin.tissoires@...il.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] HID: debug: empty output queue on read

On Wed, 17 Apr 2013, Benjamin Tissoires wrote:

> If an event occurs while the hid debugfs is forwarding events, list->tail
> is updated during copy_to_user().
> 
> Remove the gotos and use a regular while-loop to empty the queue.
> 
> Second benefit, it checks that we are not writing more than count bytes
> to the user-space output buffer.
> 
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
> ---
>  drivers/hid/hid-debug.c | 27 +++++++++++----------------
>  1 file changed, 11 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/hid/hid-debug.c b/drivers/hid/hid-debug.c
> index 094cbcf..1dc8104 100644
> --- a/drivers/hid/hid-debug.c
> +++ b/drivers/hid/hid-debug.c
> @@ -1000,6 +1000,7 @@ static ssize_t hid_debug_events_read(struct file *file, char __user *buffer,
>  		size_t count, loff_t *ppos)
>  {
>  	struct hid_debug_list *list = file->private_data;
> +	char *buf_head;
>  	int ret = 0, len;
>  	DECLARE_WAITQUEUE(wait, current);
>  
> @@ -1039,28 +1040,22 @@ static ssize_t hid_debug_events_read(struct file *file, char __user *buffer,
>  			goto out;
>  
>  		/* pass the ringbuffer contents to userspace */
> -copy_rest:
> -		if (list->tail == list->head)
> -			goto out;
> -		if (list->tail > list->head) {
> -			len = list->tail - list->head;
> +		while (list->tail != list->head && ret < count) {
> +			buf_head = &list->hid_debug_buf[list->head];
>  
> -			if (copy_to_user(buffer + ret, &list->hid_debug_buf[list->head], len)) {
> -				ret = -EFAULT;
> -				goto out;
> -			}
> -			ret += len;
> -			list->head += len;
> -		} else {
> -			len = HID_DEBUG_BUFSIZE - list->head;
> +			if (list->tail > list->head)
> +				len = list->tail - list->head;
> +			else
> +				len = HID_DEBUG_BUFSIZE - list->head;
> +
> +			len = min(count - ret, len);

This triggers a type checking warning in min(), and it seems to be correct 
on this one (mixing size_t with signed int).

drivers/hid/hid-debug.c:1079: warning: comparison of distinct pointer types lacks a cast

-- 
Jiri Kosina
SUSE Labs
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