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

Sasha Levin
2020-06-29T19:29:24Z

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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