linux-kernel - [PATCH 4.9 110/191] media: dvb_frontend: better document the -EPERM condition

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Message-Id: <20200629154007.2495120-111-sashal@kernel.org>
Date:   Mon, 29 Jun 2020 11:38:46 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH 4.9 110/191] media: dvb_frontend: better document the -EPERM condition

From: Mauro Carvalho Chehab <mchehab@...pensource.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@....samsung.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@...pensource.com>
Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 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 a7ba8e200b677..673cefb7230cb 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;
 	}
-- 
2.25.1