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Date: Wed, 5 Apr 2023 20:42:04 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Andrei Vagin <avagin@...gle.com>
Cc: Kees Cook <keescook@...omium.org>,
Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org,
Christian Brauner <brauner@...nel.org>,
Chen Yu <yu.c.chen@...el.com>, avagin@...il.com,
Dietmar Eggemann <dietmar.eggemann@....com>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Peter Oskolkov <posk@...gle.com>,
Tycho Andersen <tycho@...ho.pizza>,
Will Drewry <wad@...omium.org>,
Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 4/6] seccomp: add the synchronous mode for seccomp_unotify
On Tue, Mar 7, 2023 at 11:32 PM Andrei Vagin <avagin@...gle.com> wrote:
>
> seccomp_unotify allows more privileged processes do actions on behalf
> of less privileged processes.
>
> In many cases, the workflow is fully synchronous. It means a target
> process triggers a system call and passes controls to a supervisor
> process that handles the system call and returns controls to the target
> process. In this context, "synchronous" means that only one process is
> running and another one is waiting.
>
> There is the WF_CURRENT_CPU flag that is used to advise the scheduler to
> move the wakee to the current CPU. For such synchronous workflows, it
> makes context switches a few times faster.
>
> Right now, each interaction takes 12µs. With this patch, it takes about
> 3µs.
This is great, but:
>
> This change introduce the SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP flag that
> it used to enable the sync mode.
Other than being faster, what does this flag actually do in terms of
user-visible semantics?
>
> Signed-off-by: Andrei Vagin <avagin@...gle.com>
> ---
> include/uapi/linux/seccomp.h | 4 ++++
> kernel/seccomp.c | 31 +++++++++++++++++++++++++++++--
> 2 files changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 0fdc6ef02b94..dbfc9b37fcae 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -115,6 +115,8 @@ struct seccomp_notif_resp {
> __u32 flags;
> };
>
> +#define SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP (1UL << 0)
> +
> /* valid flags for seccomp_notif_addfd */
> #define SECCOMP_ADDFD_FLAG_SETFD (1UL << 0) /* Specify remote fd */
> #define SECCOMP_ADDFD_FLAG_SEND (1UL << 1) /* Addfd and return it, atomically */
> @@ -150,4 +152,6 @@ struct seccomp_notif_addfd {
> #define SECCOMP_IOCTL_NOTIF_ADDFD SECCOMP_IOW(3, \
> struct seccomp_notif_addfd)
>
> +#define SECCOMP_IOCTL_NOTIF_SET_FLAGS SECCOMP_IOW(4, __u64)
> +
> #endif /* _UAPI_LINUX_SECCOMP_H */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 9fca9345111c..d323edeae7da 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -143,9 +143,12 @@ struct seccomp_kaddfd {
> * filter->notify_lock.
> * @next_id: The id of the next request.
> * @notifications: A list of struct seccomp_knotif elements.
> + * @flags: A set of SECCOMP_USER_NOTIF_FD_* flags.
> */
> +
> struct notification {
> atomic_t requests;
> + u32 flags;
> u64 next_id;
> struct list_head notifications;
> };
> @@ -1117,7 +1120,10 @@ static int seccomp_do_user_notification(int this_syscall,
> INIT_LIST_HEAD(&n.addfd);
>
> atomic_inc(&match->notif->requests);
> - wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
> + if (match->notif->flags & SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP)
> + wake_up_poll_on_current_cpu(&match->wqh, EPOLLIN | EPOLLRDNORM);
> + else
> + wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
>
> /*
> * This is where we wait for a reply from userspace.
> @@ -1593,7 +1599,10 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
> knotif->error = resp.error;
> knotif->val = resp.val;
> knotif->flags = resp.flags;
> - complete(&knotif->ready);
> + if (filter->notif->flags & SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP)
> + complete_on_current_cpu(&knotif->ready);
> + else
> + complete(&knotif->ready);
> out:
> mutex_unlock(&filter->notify_lock);
> return ret;
> @@ -1623,6 +1632,22 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
> return ret;
> }
>
> +static long seccomp_notify_set_flags(struct seccomp_filter *filter,
> + unsigned long flags)
> +{
> + long ret;
> +
> + if (flags & ~SECCOMP_USER_NOTIF_FD_SYNC_WAKE_UP)
> + return -EINVAL;
> +
> + ret = mutex_lock_interruptible(&filter->notify_lock);
> + if (ret < 0)
> + return ret;
> + filter->notif->flags = flags;
> + mutex_unlock(&filter->notify_lock);
> + return 0;
> +}
> +
> static long seccomp_notify_addfd(struct seccomp_filter *filter,
> struct seccomp_notif_addfd __user *uaddfd,
> unsigned int size)
> @@ -1752,6 +1777,8 @@ static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
> case SECCOMP_IOCTL_NOTIF_ID_VALID_WRONG_DIR:
> case SECCOMP_IOCTL_NOTIF_ID_VALID:
> return seccomp_notify_id_valid(filter, buf);
> + case SECCOMP_IOCTL_NOTIF_SET_FLAGS:
> + return seccomp_notify_set_flags(filter, arg);
> }
>
> /* Extensible Argument ioctls */
> --
> 2.40.0.rc0.216.gc4246ad0f0-goog
>
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