io_uring: add IORING_OP_WAITID support

This adds support for an async version of waitid(2), in a fully async
version. If an event isn't immediately available, wait for a callback
to trigger a retry.

The format of the sqe is as follows:

sqe->len		The 'which', the idtype being queried/waited for.
sqe->fd			The 'pid' (or id) being waited for.
sqe->file_index		The 'options' being set.
sqe->addr2		A pointer to siginfo_t, if any, being filled in.

buf_index, add3, and waitid_flags are reserved/unused for now.
waitid_flags will be used for options for this request type. One
interesting use case may be to add multi-shot support, so that the
request stays armed and posts a notification every time a monitored
process state change occurs.

Note that this does not support rusage, on Arnd's recommendation.

See the waitid(2) man page for details on the arguments.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 372 insertions, 0 deletions

diff --git a/io_uring/waitid.c b/io_uring/waitid.c
new file mode 100644
index 000000000000..6f851978606d
--- /dev/null
+++ b/io_uring/waitid.c
@@ -0,0 +1,372 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Support for async notification of waitid
+ */
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include <linux/compat.h>
+#include <linux/io_uring.h>
+
+#include <uapi/linux/io_uring.h>
+
+#include "io_uring.h"
+#include "cancel.h"
+#include "waitid.h"
+#include "../kernel/exit.h"
+
+static void io_waitid_cb(struct io_kiocb *req, struct io_tw_state *ts);
+
+#define IO_WAITID_CANCEL_FLAG	BIT(31)
+#define IO_WAITID_REF_MASK	GENMASK(30, 0)
+
+struct io_waitid {
+	struct file *file;
+	int which;
+	pid_t upid;
+	int options;
+	atomic_t refs;
+	struct wait_queue_head *head;
+	struct siginfo __user *infop;
+	struct waitid_info info;
+};
+
+static void io_waitid_free(struct io_kiocb *req)
+{
+	struct io_waitid_async *iwa = req->async_data;
+
+	put_pid(iwa->wo.wo_pid);
+	kfree(req->async_data);
+	req->async_data = NULL;
+	req->flags &= ~REQ_F_ASYNC_DATA;
+}
+
+#ifdef CONFIG_COMPAT
+static bool io_waitid_compat_copy_si(struct io_waitid *iw, int signo)
+{
+	struct compat_siginfo __user *infop;
+	bool ret;
+
+	infop = (struct compat_siginfo __user *) iw->infop;
+
+	if (!user_write_access_begin(infop, sizeof(*infop)))
+		return false;
+
+	unsafe_put_user(signo, &infop->si_signo, Efault);
+	unsafe_put_user(0, &infop->si_errno, Efault);
+	unsafe_put_user(iw->info.cause, &infop->si_code, Efault);
+	unsafe_put_user(iw->info.pid, &infop->si_pid, Efault);
+	unsafe_put_user(iw->info.uid, &infop->si_uid, Efault);
+	unsafe_put_user(iw->info.status, &infop->si_status, Efault);
+	ret = true;
+done:
+	user_write_access_end();
+	return ret;
+Efault:
+	ret = false;
+	goto done;
+}
+#endif
+
+static bool io_waitid_copy_si(struct io_kiocb *req, int signo)
+{
+	struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+	bool ret;
+
+	if (!iw->infop)
+		return true;
+
+#ifdef CONFIG_COMPAT
+	if (req->ctx->compat)
+		return io_waitid_compat_copy_si(iw, signo);
+#endif
+
+	if (!user_write_access_begin(iw->infop, sizeof(*iw->infop)))
+		return false;
+
+	unsafe_put_user(signo, &iw->infop->si_signo, Efault);
+	unsafe_put_user(0, &iw->infop->si_errno, Efault);
+	unsafe_put_user(iw->info.cause, &iw->infop->si_code, Efault);
+	unsafe_put_user(iw->info.pid, &iw->infop->si_pid, Efault);
+	unsafe_put_user(iw->info.uid, &iw->infop->si_uid, Efault);
+	unsafe_put_user(iw->info.status, &iw->infop->si_status, Efault);
+	ret = true;
+done:
+	user_write_access_end();
+	return ret;
+Efault:
+	ret = false;
+	goto done;
+}
+
+static int io_waitid_finish(struct io_kiocb *req, int ret)
+{
+	int signo = 0;
+
+	if (ret > 0) {
+		signo = SIGCHLD;
+		ret = 0;
+	}
+
+	if (!io_waitid_copy_si(req, signo))
+		ret = -EFAULT;
+	io_waitid_free(req);
+	return ret;
+}
+
+static void io_waitid_complete(struct io_kiocb *req, int ret)
+{
+	struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+	struct io_tw_state ts = { .locked = true };
+
+	/* anyone completing better be holding a reference */
+	WARN_ON_ONCE(!(atomic_read(&iw->refs) & IO_WAITID_REF_MASK));
+
+	lockdep_assert_held(&req->ctx->uring_lock);
+
+	/*
+	 * Did cancel find it meanwhile?
+	 */
+	if (hlist_unhashed(&req->hash_node))
+		return;
+
+	hlist_del_init(&req->hash_node);
+
+	ret = io_waitid_finish(req, ret);
+	if (ret < 0)
+		req_set_fail(req);
+	io_req_set_res(req, ret, 0);
+	io_req_task_complete(req, &ts);
+}
+
+static bool __io_waitid_cancel(struct io_ring_ctx *ctx, struct io_kiocb *req)
+{
+	struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+	struct io_waitid_async *iwa = req->async_data;
+
+	/*
+	 * Mark us canceled regardless of ownership. This will prevent a
+	 * potential retry from a spurious wakeup.
+	 */
+	atomic_or(IO_WAITID_CANCEL_FLAG, &iw->refs);
+
+	/* claim ownership */
+	if (atomic_fetch_inc(&iw->refs) & IO_WAITID_REF_MASK)
+		return false;
+
+	spin_lock_irq(&iw->head->lock);
+	list_del_init(&iwa->wo.child_wait.entry);
+	spin_unlock_irq(&iw->head->lock);
+	io_waitid_complete(req, -ECANCELED);
+	return true;
+}
+
+int io_waitid_cancel(struct io_ring_ctx *ctx, struct io_cancel_data *cd,
+		     unsigned int issue_flags)
+{
+	struct hlist_node *tmp;
+	struct io_kiocb *req;
+	int nr = 0;
+
+	if (cd->flags & (IORING_ASYNC_CANCEL_FD|IORING_ASYNC_CANCEL_FD_FIXED))
+		return -ENOENT;
+
+	io_ring_submit_lock(ctx, issue_flags);
+	hlist_for_each_entry_safe(req, tmp, &ctx->waitid_list, hash_node) {
+		if (req->cqe.user_data != cd->data &&
+		    !(cd->flags & IORING_ASYNC_CANCEL_ANY))
+			continue;
+		if (__io_waitid_cancel(ctx, req))
+			nr++;
+		if (!(cd->flags & IORING_ASYNC_CANCEL_ALL))
+			break;
+	}
+	io_ring_submit_unlock(ctx, issue_flags);
+
+	if (nr)
+		return nr;
+
+	return -ENOENT;
+}
+
+bool io_waitid_remove_all(struct io_ring_ctx *ctx, struct task_struct *task,
+			  bool cancel_all)
+{
+	struct hlist_node *tmp;
+	struct io_kiocb *req;
+	bool found = false;
+
+	lockdep_assert_held(&ctx->uring_lock);
+
+	hlist_for_each_entry_safe(req, tmp, &ctx->waitid_list, hash_node) {
+		if (!io_match_task_safe(req, task, cancel_all))
+			continue;
+		__io_waitid_cancel(ctx, req);
+		found = true;
+	}
+
+	return found;
+}
+
+static inline bool io_waitid_drop_issue_ref(struct io_kiocb *req)
+{
+	struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+	struct io_waitid_async *iwa = req->async_data;
+
+	if (!atomic_sub_return(1, &iw->refs))
+		return false;
+
+	/*
+	 * Wakeup triggered, racing with us. It was prevented from
+	 * completing because of that, queue up the tw to do that.
+	 */
+	req->io_task_work.func = io_waitid_cb;
+	io_req_task_work_add(req);
+	remove_wait_queue(iw->head, &iwa->wo.child_wait);
+	return true;
+}
+
+static void io_waitid_cb(struct io_kiocb *req, struct io_tw_state *ts)
+{
+	struct io_waitid_async *iwa = req->async_data;
+	struct io_ring_ctx *ctx = req->ctx;
+	int ret;
+
+	io_tw_lock(ctx, ts);
+
+	ret = __do_wait(&iwa->wo);
+
+	/*
+	 * If we get -ERESTARTSYS here, we need to re-arm and check again
+	 * to ensure we get another callback. If the retry works, then we can
+	 * just remove ourselves from the waitqueue again and finish the
+	 * request.
+	 */
+	if (unlikely(ret == -ERESTARTSYS)) {
+		struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+
+		/* Don't retry if cancel found it meanwhile */
+		ret = -ECANCELED;
+		if (!(atomic_read(&iw->refs) & IO_WAITID_CANCEL_FLAG)) {
+			iw->head = &current->signal->wait_chldexit;
+			add_wait_queue(iw->head, &iwa->wo.child_wait);
+			ret = __do_wait(&iwa->wo);
+			if (ret == -ERESTARTSYS) {
+				/* retry armed, drop our ref */
+				io_waitid_drop_issue_ref(req);
+				return;
+			}
+
+			remove_wait_queue(iw->head, &iwa->wo.child_wait);
+		}
+	}
+
+	io_waitid_complete(req, ret);
+}
+
+static int io_waitid_wait(struct wait_queue_entry *wait, unsigned mode,
+			  int sync, void *key)
+{
+	struct wait_opts *wo = container_of(wait, struct wait_opts, child_wait);
+	struct io_waitid_async *iwa = container_of(wo, struct io_waitid_async, wo);
+	struct io_kiocb *req = iwa->req;
+	struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+	struct task_struct *p = key;
+
+	if (!pid_child_should_wake(wo, p))
+		return 0;
+
+	/* cancel is in progress */
+	if (atomic_fetch_inc(&iw->refs) & IO_WAITID_REF_MASK)
+		return 1;
+
+	req->io_task_work.func = io_waitid_cb;
+	io_req_task_work_add(req);
+	list_del_init(&wait->entry);
+	return 1;
+}
+
+int io_waitid_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
+{
+	struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+
+	if (sqe->addr || sqe->buf_index || sqe->addr3 || sqe->waitid_flags)
+		return -EINVAL;
+
+	iw->which = READ_ONCE(sqe->len);
+	iw->upid = READ_ONCE(sqe->fd);
+	iw->options = READ_ONCE(sqe->file_index);
+	iw->infop = u64_to_user_ptr(READ_ONCE(sqe->addr2));
+	return 0;
+}
+
+int io_waitid(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_waitid *iw = io_kiocb_to_cmd(req, struct io_waitid);
+	struct io_ring_ctx *ctx = req->ctx;
+	struct io_waitid_async *iwa;
+	int ret;
+
+	if (io_alloc_async_data(req))
+		return -ENOMEM;
+
+	iwa = req->async_data;
+	iwa->req = req;
+
+	ret = kernel_waitid_prepare(&iwa->wo, iw->which, iw->upid, &iw->info,
+					iw->options, NULL);
+	if (ret)
+		goto done;
+
+	/*
+	 * Mark the request as busy upfront, in case we're racing with the
+	 * wakeup. If we are, then we'll notice when we drop this initial
+	 * reference again after arming.
+	 */
+	atomic_set(&iw->refs, 1);
+
+	/*
+	 * Cancel must hold the ctx lock, so there's no risk of cancelation
+	 * finding us until a) we remain on the list, and b) the lock is
+	 * dropped. We only need to worry about racing with the wakeup
+	 * callback.
+	 */
+	io_ring_submit_lock(ctx, issue_flags);
+	hlist_add_head(&req->hash_node, &ctx->waitid_list);
+
+	init_waitqueue_func_entry(&iwa->wo.child_wait, io_waitid_wait);
+	iwa->wo.child_wait.private = req->task;
+	iw->head = &current->signal->wait_chldexit;
+	add_wait_queue(iw->head, &iwa->wo.child_wait);
+
+	ret = __do_wait(&iwa->wo);
+	if (ret == -ERESTARTSYS) {
+		/*
+		 * Nobody else grabbed a reference, it'll complete when we get
+		 * a waitqueue callback, or if someone cancels it.
+		 */
+		if (!io_waitid_drop_issue_ref(req)) {
+			io_ring_submit_unlock(ctx, issue_flags);
+			return IOU_ISSUE_SKIP_COMPLETE;
+		}
+
+		/*
+		 * Wakeup triggered, racing with us. It was prevented from
+		 * completing because of that, queue up the tw to do that.
+		 */
+		io_ring_submit_unlock(ctx, issue_flags);
+		return IOU_ISSUE_SKIP_COMPLETE;
+	}
+
+	hlist_del_init(&req->hash_node);
+	remove_wait_queue(iw->head, &iwa->wo.child_wait);
+	ret = io_waitid_finish(req, ret);
+
+	io_ring_submit_unlock(ctx, issue_flags);
+done:
+	if (ret < 0)
+		req_set_fail(req);
+	io_req_set_res(req, ret, 0);
+	return IOU_OK;
+}