Re: [PATCH net-next v7 4/4] eventpoll: Add epoll ioctl for epoll_params

[Eric Dumazet]

On Fri, Feb 9, 2024 at 10:15 PM Joe Damato <jdamato@xxxxxxxxxx> wrote:
>
> Add an ioctl for getting and setting epoll_params. User programs can use
> this ioctl to get and set the busy poll usec time, packet budget, and
> prefer busy poll params for a specific epoll context.
>
> Parameters are limited:
>   - busy_poll_usecs is limited to <= s32_max
>   - busy_poll_budget is limited to <= NAPI_POLL_WEIGHT by unprivileged
>     users (!capable(CAP_NET_ADMIN))
>   - prefer_busy_poll must be 0 or 1
>   - __pad must be 0
>
> Signed-off-by: Joe Damato <jdamato@xxxxxxxxxx>
> Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> Reviewed-by: Jiri Slaby <jirislaby@xxxxxxxxxx>
> ---
>  .../userspace-api/ioctl/ioctl-number.rst      |  1 +
>  fs/eventpoll.c                                | 72 +++++++++++++++++++
>  include/uapi/linux/eventpoll.h                | 13 ++++
>  3 files changed, 86 insertions(+)
>
> diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> index 457e16f06e04..b33918232f78 100644
> --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> @@ -309,6 +309,7 @@ Code  Seq#    Include File                                           Comments
>  0x89  0B-DF  linux/sockios.h
>  0x89  E0-EF  linux/sockios.h                                         SIOCPROTOPRIVATE range
>  0x89  F0-FF  linux/sockios.h                                         SIOCDEVPRIVATE range
> +0x8A  00-1F  linux/eventpoll.h
>  0x8B  all    linux/wireless.h
>  0x8C  00-3F                                                          WiNRADiO driver
>                                                                       <http://www.winradio.com.au/>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 1b8d01af0c2c..aa58d42737e6 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -37,6 +37,7 @@
>  #include <linux/seq_file.h>
>  #include <linux/compat.h>
>  #include <linux/rculist.h>
> +#include <linux/capability.h>
>  #include <net/busy_poll.h>
>
>  /*
> @@ -494,6 +495,49 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
>         ep->napi_id = napi_id;
>  }
>
> +static long ep_eventpoll_bp_ioctl(struct file *file, unsigned int cmd,
> +                                 unsigned long arg)
> +{
> +       struct eventpoll *ep = file->private_data;
> +       void __user *uarg = (void __user *)arg;
> +       struct epoll_params epoll_params;
> +
> +       switch (cmd) {
> +       case EPIOCSPARAMS:
> +               if (copy_from_user(&epoll_params, uarg, sizeof(epoll_params)))
> +                       return -EFAULT;
> +
> +               /* pad byte must be zero */
> +               if (epoll_params.__pad)
> +                       return -EINVAL;
> +
> +               if (epoll_params.busy_poll_usecs > S32_MAX)
> +                       return -EINVAL;
> +
> +               if (epoll_params.prefer_busy_poll > 1)
> +                       return -EINVAL;
> +
> +               if (epoll_params.busy_poll_budget > NAPI_POLL_WEIGHT &&
> +                   !capable(CAP_NET_ADMIN))
> +                       return -EPERM;
> +
> +               ep->busy_poll_usecs = epoll_params.busy_poll_usecs;

You need WRITE_ONCE(ep->XXX, val); for all these settings.

> +               ep->busy_poll_budget = epoll_params.busy_poll_budget;
> +               ep->prefer_busy_poll = epoll_params.prefer_busy_poll;
> +               return 0;
> +       case EPIOCGPARAMS:
> +               memset(&epoll_params, 0, sizeof(epoll_params));
> +               epoll_params.busy_poll_usecs = ep->busy_poll_usecs;

You need to use READ_ONCE(ep->XXXXX) for the three reads.


> +               epoll_params.busy_poll_budget = ep->busy_poll_budget;
> +               epoll_params.prefer_busy_poll = ep->prefer_busy_poll;
> +               if (copy_to_user(uarg, &epoll_params, sizeof(epoll_params)))
> +                       return -EFAULT;
> +               return 0;
> +       default:
> +               return -ENOIOCTLCMD;
> +       }
> +}
>