[PATCH stable 4.9 13/21] media: dvb_frontend: better document the -EPERM condition

From: Florian Fainelli <f.fainelli@gmail.com>
To: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org,
	Mauro Carvalho Chehab <mchehab@s-opensource.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Michael Krufky <mkrufky@linuxtv.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Shuah Khan <shuah@kernel.org>,
	Jaedon Shin <jaedon.shin@gmail.com>,
	Colin Ian King <colin.king@canonical.com>,
	Katsuhiro Suzuki <suzuki.katsuhiro@socionext.com>,
	Satendra Singh Thakur <satendra.t@samsung.com>,
	linux-media@vger.kernel.org (open list:MEDIA INPUT
	INFRASTRUCTURE (V4L/DVB)),
	linux-fsdevel@vger.kernel.org (open list:FILESYSTEMS (VFS and
	infrastructure))
Subject: [PATCH stable 4.9 13/21] media: dvb_frontend: better document the -EPERM condition
Date: Fri,  5 Jun 2020 09:25:10 -0700	[thread overview]
Message-ID: <20200605162518.28099-14-florian.fainelli@broadcom.com> (raw)
In-Reply-To: <20200605162518.28099-1-florian.fainelli@broadcom.com>

From: Mauro Carvalho Chehab <mchehab@s-opensource.com>

commit da5516b5e81d45a96291823620f6c820178dc055 upstream

Two readonly ioctls can't be allowed if the frontend device
is opened in read only mode. Explain why.

Reviewed by: Shuah Khan <shuahkh@osg.samsung.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 drivers/media/dvb-core/dvb_frontend.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/media/dvb-core/dvb_frontend.c b/drivers/media/dvb-core/dvb_frontend.c
index a7ba8e200b67..673cefb7230c 100644
--- a/drivers/media/dvb-core/dvb_frontend.c
+++ b/drivers/media/dvb-core/dvb_frontend.c
@@ -1925,9 +1925,23 @@ static int dvb_frontend_ioctl(struct file *file, unsigned int cmd, void *parg)
 		return -ENODEV;
 	}
 
-	if ((file->f_flags & O_ACCMODE) == O_RDONLY &&
-	    (_IOC_DIR(cmd) != _IOC_READ || cmd == FE_GET_EVENT ||
-	     cmd == FE_DISEQC_RECV_SLAVE_REPLY)) {
+	/*
+	 * If the frontend is opened in read-only mode, only the ioctls
+	 * that don't interfere with the tune logic should be accepted.
+	 * That allows an external application to monitor the DVB QoS and
+	 * statistics parameters.
+	 *
+	 * That matches all _IOR() ioctls, except for two special cases:
+	 *   - FE_GET_EVENT is part of the tuning logic on a DVB application;
+	 *   - FE_DISEQC_RECV_SLAVE_REPLY is part of DiSEqC 2.0
+	 *     setup
+	 * So, those two ioctls should also return -EPERM, as otherwise
+	 * reading from them would interfere with a DVB tune application
+	 */
+	if ((file->f_flags & O_ACCMODE) == O_RDONLY
+	    && (_IOC_DIR(cmd) != _IOC_READ
+		|| cmd == FE_GET_EVENT
+		|| cmd == FE_DISEQC_RECV_SLAVE_REPLY)) {
 		up(&fepriv->sem);
 		return -EPERM;
 	}
-- 
2.17.1

