Re: [PATCH v3] usb: gadget: ffs: Fix sparse error

[Michal Nazarewicz <mina86@mina86.com>]

On Wed, Dec 24 2014, Rohith Seelaboyina wrote:
> This patch fixes the sparse error in functionfs
> driver.
>
> drivers/usb/gadget/function/f_fs.c:400:44: error: bad
> constant experssion.
>
> Dynamic memory allocation through kcalloc is more safer
> than declaring variable array size, Fix this error by
> using kcalloc for memory allocation, Check if memory
> allocation is successful and return -ENOMEM on failure.
>
> Signed-off-by: Rohith Seelaboyina <rseelaboyina@nvidia.com>

This has already been addressed in a way that does not require dynamic
allocation: <https://lkml.org/lkml/2014/9/10/513>.  I'd rather patch
attached at the end would be applied.

> ---
>  drivers/usb/gadget/function/f_fs.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index 63314ede7ba6..8af3f7906c83 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -397,10 +397,13 @@ static ssize_t __ffs_ep0_read_events(struct ffs_data *ffs, char __user *buf,
>  	 * We are holding ffs->ev.waitq.lock and ffs->mutex and we need
>  	 * to release them.
>  	 */
> -	struct usb_functionfs_event events[n];
>  	unsigned i = 0;
> +	int ret;
> +	struct usb_functionfs_event *events;
>  
> -	memset(events, 0, sizeof events);
> +	events = kcalloc(n, sizeof(*events), GFP_KERNEL);
> +	if (!events)
> +		return -ENOMEM;
>  
>  	do {
>  		events[i].type = ffs->ev.types[i];
> @@ -421,8 +424,10 @@ static ssize_t __ffs_ep0_read_events(struct ffs_data *ffs, char __user *buf,
>  	spin_unlock_irq(&ffs->ev.waitq.lock);
>  	mutex_unlock(&ffs->mutex);
>  
> -	return unlikely(__copy_to_user(buf, events, sizeof events))
> -		? -EFAULT : sizeof events;
> +	ret = unlikely(__copy_to_user(buf, events, n * sizeof(*events)))
> +		? -EFAULT : n * sizeof(*events);
> +	kfree(events);
> +	return ret;
>  }
>  
>  static ssize_t ffs_ep0_read(struct file *file, char __user *buf,
> -- 
> 1.9.1
>

>From 8c813b2563cae23c69895833972da1c1ee92d8dd Mon Sep 17 00:00:00 2001
From: Michal Nazarewicz <mina86@mina86.com>
Date: Wed, 10 Sep 2014 17:50:24 +0200
Subject: [PATCHv2] usb: f_fs: refactor and document __ffs_ep0_read_events better

Instead of using variable length array, use a static length equal to
the size of the ffs->ev.types array.  This gets rid of a sparse warning:

	drivers/usb/gadget/function/f_fs.c:401:44: warning:
	Variable length array is used.

and makes it more explicit that the array has a very tight upper size
limit.  Also add some more documentation about the ev.types array and
how its size is limited and affects the rest of the code.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
---
 drivers/usb/gadget/function/f_fs.c | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 63314ed..a00ee97 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -390,17 +390,20 @@ done_spin:
 	return ret;
 }
 
+/* Called with ffs->ev.waitq.lock and ffs->mutex held, both released on exit. */
 static ssize_t __ffs_ep0_read_events(struct ffs_data *ffs, char __user *buf,
 				     size_t n)
 {
 	/*
-	 * We are holding ffs->ev.waitq.lock and ffs->mutex and we need
-	 * to release them.
+	 * n cannot be bigger than ffs->ev.count, which cannot be bigger than
+	 * size of ffs->ev.types array (which is four) so that's how much space
+	 * we reserve.
 	 */
-	struct usb_functionfs_event events[n];
+	struct usb_functionfs_event events[ARRAY_SIZE(ffs->ev.types)];
+	const size_t size = n * sizeof *events;
 	unsigned i = 0;
 
-	memset(events, 0, sizeof events);
+	memset(events, 0, size);
 
 	do {
 		events[i].type = ffs->ev.types[i];
@@ -410,19 +413,15 @@ static ssize_t __ffs_ep0_read_events(struct ffs_data *ffs, char __user *buf,
 		}
 	} while (++i < n);
 
-	if (n < ffs->ev.count) {
-		ffs->ev.count -= n;
+	ffs->ev.count -= n;
+	if (ffs->ev.count)
 		memmove(ffs->ev.types, ffs->ev.types + n,
 			ffs->ev.count * sizeof *ffs->ev.types);
-	} else {
-		ffs->ev.count = 0;
-	}
 
 	spin_unlock_irq(&ffs->ev.waitq.lock);
 	mutex_unlock(&ffs->mutex);
 
-	return unlikely(__copy_to_user(buf, events, sizeof events))
-		? -EFAULT : sizeof events;
+	return unlikely(__copy_to_user(buf, events, size)) ? -EFAULT : size;
 }
 
 static ssize_t ffs_ep0_read(struct file *file, char __user *buf,
@@ -2377,6 +2376,13 @@ static void __ffs_event_add(struct ffs_data *ffs,
 	if (ffs->setup_state == FFS_SETUP_PENDING)
 		ffs->setup_state = FFS_SETUP_CANCELLED;
 
+	/*
+	 * Logic of this function guarantees that there are at most four pending
+	 * evens on ffs->ev.types queue.  This is important because the queue
+	 * has space for four elements only and __ffs_ep0_read_events function
+	 * depends on that limit as well.  If more event types are added, those
+	 * limits have to be revisited or guaranteed to still hold.
+	 */
 	switch (type) {
 	case FUNCTIONFS_RESUME:
 		rem_type2 = FUNCTIONFS_SUSPEND;
-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
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