linux-kernel - [RFC PATCH] misc: xilinx-sdfec: Check if file->private

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Date:   Thu, 20 May 2021 10:01:50 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Derek Kiernan <derek.kiernan@...inx.com>
Cc:     Dragan Cvetic <dragan.cvetic@...inx.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michal Simek <michal.simek@...inx.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Guenter Roeck <linux@...ck-us.net>
Subject: [RFC PATCH] misc: xilinx-sdfec: Check if file->private_data is NULL

container_of() only returns NULL if the passed pointer is NULL _and_
the embedded element is the first element of the structure. Even if that
is the case, testing against it is misleading and possibly dangerous
because the position of the embedded element may change. Explicitly
check if the parameter is NULL and bail out if so instead of checking
the result of container_of().

Signed-off-by: Guenter Roeck <linux@...ck-us.net>
---
RFC:

The NULL check in the poll function is likely unnecessary. Interestingly,
there is no NULL check in the ioctl function, even though there is a
similar container_of() in that function. However, I do not feel
comfortable enough to change the functionality of this code and drop
the check entirely.

 drivers/misc/xilinx_sdfec.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/xilinx_sdfec.c b/drivers/misc/xilinx_sdfec.c
index 23c8448a9c3b..0a3721d31dea 100644
--- a/drivers/misc/xilinx_sdfec.c
+++ b/drivers/misc/xilinx_sdfec.c
@@ -1011,11 +1011,11 @@ static __poll_t xsdfec_poll(struct file *file, poll_table *wait)
 	__poll_t mask = 0;
 	struct xsdfec_dev *xsdfec;

-	xsdfec = container_of(file->private_data, struct xsdfec_dev, miscdev);
-
-	if (!xsdfec)
+	if (!file->private_data)
 		return EPOLLNVAL | EPOLLHUP;

+	xsdfec = container_of(file->private_data, struct xsdfec_dev, miscdev);
+
 	poll_wait(file, &xsdfec->waitq, wait);

 	/* XSDFEC ISR detected an error */
-- 
2.25.1