linux-kernel - [PATCH 4.9 018/116] gpio: chardev: Return error for seek operations

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Date:   Fri,  6 Jan 2017 22:43:00 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH 4.9 018/116] gpio: chardev: Return error for seek operations

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Lars-Peter Clausen <lars@...afoo.de>

commit f4e81c529767b9a33d1b27695c54dc84a14af30d upstream.

The GPIO chardev is used for management tasks (allocating line and event
handles) and does neither support read() nor write() operations. Hence it
does not make much sense to allow seek operations.

Currently the chardev uses noop_llseek() for its seek implementation. This
function does not move the pointer and simply returns the current position
(always 0 for the GPIO chardev). noop_llseek() is primarily meant for
devices that can not support seek, but where there might be a user that
depends on the seek() operation succeeding. For newly added devices that
can not support seek operations it is recommended to use no_llseek(), which
will return an error. For more information see commit 6038f373a3dc
("llseek: automatically add .llseek fop").

Unfortunately this was overlooked when the GPIO chardev ABI was introduced.
But it is highly unlikely that since then userspace applications have
appeared that rely on being able to perform non-failing seek operations on
a GPIO chardev file descriptor. So it should be safe to change from
noop_llseel() to no_seek(). Also use nonseekable_open() in the chardev
open() callback to clear the FMODE_SEEK, FMODE_PREAD and FMODE_PWRITE flags
from the file. Neither of these should be set on a file that does not
support seek operations.

Fixes: 3c702e9987e2 ("gpio: add a userspace chardev ABI for GPIOs")
Signed-off-by: Lars-Peter Clausen <lars@...afoo.de>
Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/gpio/gpiolib.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -986,7 +986,8 @@ static int gpio_chrdev_open(struct inode
 		return -ENODEV;
 	get_device(&gdev->dev);
 	filp->private_data = gdev;
-	return 0;
+
+	return nonseekable_open(inode, filp);
 }

 /**
@@ -1011,7 +1012,7 @@ static const struct file_operations gpio
 	.release = gpio_chrdev_release,
 	.open = gpio_chrdev_open,
 	.owner = THIS_MODULE,
-	.llseek = noop_llseek,
+	.llseek = no_llseek,
 	.unlocked_ioctl = gpio_ioctl,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl = gpio_ioctl_compat,