Modify pipe_poll() to properly check for pending direct writes.

With r349546, it is a responsibility of the writer to clear PIPE_DIRECTW
after pinned data has been read.  In particular, once a reader has
drained this data, there is a small window where the pipe is empty but
PIPE_DIRECTW is set.  pipe_poll() was using the presence of PIPE_DIRECTW
to determine whether to return POLLIN, so in this window it would
claim that data was available to read when this was not the case.

Fix this by modifying several checks for PIPE_DIRECTW to instead look
at the number of residual bytes in data pinned by a direct writer.  In
some cases we really do want to check for PIPE_DIRECTW, since the
presence of this flag indicates that any attempt to write to the pipe
will block on the existing direct writer.

Bisected and test case provided by:	mav
Tested by:	pho
Reviewed by:	kib
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D21333

1 files changed, 24 insertions, 28 deletions

diff --git a/sys/kern/sys_pipe.c b/sys/kern/sys_pipe.c
index e516e277ee18..df0d027502c9 100644
--- a/sys/kern/sys_pipe.c
+++ b/sys/kern/sys_pipe.c
@@ -709,11 +709,9 @@ pipe_read(struct file *fp, struct uio *uio, struct ucred *active_cred,
 		/*
 		 * Direct copy, bypassing a kernel buffer.
 		 */
-		} else if ((size = rpipe->pipe_map.cnt) &&
-			   (rpipe->pipe_state & PIPE_DIRECTW)) {
+		} else if ((size = rpipe->pipe_map.cnt) != 0) {
 			if (size > uio->uio_resid)
 				size = (u_int) uio->uio_resid;
-
 			PIPE_UNLOCK(rpipe);
 			error = uiomove_fromphys(rpipe->pipe_map.ms,
 			    rpipe->pipe_map.pos, size, uio);
@@ -819,32 +817,33 @@ pipe_build_write_buffer(struct pipe *wpipe, struct uio *uio)
 	u_int size;
 	int i;
 
-	PIPE_LOCK_ASSERT(wpipe, MA_NOTOWNED);
-	KASSERT(wpipe->pipe_state & PIPE_DIRECTW,
-		("Clone attempt on non-direct write pipe!"));
+	PIPE_LOCK_ASSERT(wpipe, MA_OWNED);
+	KASSERT((wpipe->pipe_state & PIPE_DIRECTW) == 0,
+	    ("%s: PIPE_DIRECTW set on %p", __func__, wpipe));
+	KASSERT(wpipe->pipe_map.cnt == 0,
+	    ("%s: pipe map for %p contains residual data", __func__, wpipe));
 
 	if (uio->uio_iov->iov_len > wpipe->pipe_buffer.size)
                 size = wpipe->pipe_buffer.size;
 	else
                 size = uio->uio_iov->iov_len;
 
-	if ((i = vm_fault_quick_hold_pages(&curproc->p_vmspace->vm_map,
+	wpipe->pipe_state |= PIPE_DIRECTW;
+	PIPE_UNLOCK(wpipe);
+	i = vm_fault_quick_hold_pages(&curproc->p_vmspace->vm_map,
 	    (vm_offset_t)uio->uio_iov->iov_base, size, VM_PROT_READ,
-	    wpipe->pipe_map.ms, PIPENPAGES)) < 0)
+	    wpipe->pipe_map.ms, PIPENPAGES);
+	PIPE_LOCK(wpipe);
+	if (i < 0) {
+		wpipe->pipe_state &= ~PIPE_DIRECTW;
 		return (EFAULT);
+	}
 
-/*
- * set up the control block
- */
 	wpipe->pipe_map.npages = i;
 	wpipe->pipe_map.pos =
 	    ((vm_offset_t) uio->uio_iov->iov_base) & PAGE_MASK;
 	wpipe->pipe_map.cnt = size;
 
-/*
- * and update the uio data
- */
-
 	uio->uio_iov->iov_len -= size;
 	uio->uio_iov->iov_base = (char *)uio->uio_iov->iov_base + size;
 	if (uio->uio_iov->iov_len == 0)
@@ -864,6 +863,8 @@ pipe_destroy_write_buffer(struct pipe *wpipe)
 	PIPE_LOCK_ASSERT(wpipe, MA_OWNED);
 	KASSERT((wpipe->pipe_state & PIPE_DIRECTW) != 0,
 	    ("%s: PIPE_DIRECTW not set on %p", __func__, wpipe));
+	KASSERT(wpipe->pipe_map.cnt == 0,
+	    ("%s: pipe map for %p contains residual data", __func__, wpipe));
 
 	wpipe->pipe_state &= ~PIPE_DIRECTW;
 	vm_page_unhold_pages(wpipe->pipe_map.ms, wpipe->pipe_map.npages);
@@ -889,6 +890,7 @@ pipe_clone_write_buffer(struct pipe *wpipe)
 
 	size = wpipe->pipe_map.cnt;
 	pos = wpipe->pipe_map.pos;
+	wpipe->pipe_map.cnt = 0;
 
 	wpipe->pipe_buffer.in = size;
 	wpipe->pipe_buffer.out = 0;
@@ -946,7 +948,6 @@ retry:
 		else
 			goto retry;
 	}
-	wpipe->pipe_map.cnt = 0;	/* transfer not ready yet */
 	if (wpipe->pipe_buffer.cnt > 0) {
 		if (wpipe->pipe_state & PIPE_WANTR) {
 			wpipe->pipe_state &= ~PIPE_WANTR;
@@ -963,19 +964,15 @@ retry:
 			goto retry;
 	}
 
-	wpipe->pipe_state |= PIPE_DIRECTW;
-
-	PIPE_UNLOCK(wpipe);
 	error = pipe_build_write_buffer(wpipe, uio);
-	PIPE_LOCK(wpipe);
 	if (error) {
-		wpipe->pipe_state &= ~PIPE_DIRECTW;
 		pipeunlock(wpipe);
 		goto error1;
 	}
 
 	while (wpipe->pipe_map.cnt != 0) {
 		if (wpipe->pipe_state & PIPE_EOF) {
+			wpipe->pipe_map.cnt = 0;
 			pipe_destroy_write_buffer(wpipe);
 			pipeselwakeup(wpipe);
 			pipeunlock(wpipe);
@@ -1129,7 +1126,7 @@ pipe_write(struct file *fp, struct uio *uio, struct ucred *active_cred,
 		 * pipe buffer.  We break out if a signal occurs or the
 		 * reader goes away.
 		 */
-		if (wpipe->pipe_state & PIPE_DIRECTW) {
+		if (wpipe->pipe_map.cnt != 0) {
 			if (wpipe->pipe_state & PIPE_WANTR) {
 				wpipe->pipe_state &= ~PIPE_WANTR;
 				wakeup(wpipe);
@@ -1347,7 +1344,7 @@ pipe_ioctl(struct file *fp, u_long cmd, void *data, struct ucred *active_cred,
 			PIPE_UNLOCK(mpipe);
 			return (0);
 		}
-		if (mpipe->pipe_state & PIPE_DIRECTW)
+		if (mpipe->pipe_map.cnt != 0)
 			*(int *)data = mpipe->pipe_map.cnt;
 		else
 			*(int *)data = mpipe->pipe_buffer.cnt;
@@ -1403,14 +1400,13 @@ pipe_poll(struct file *fp, int events, struct ucred *active_cred,
 		goto locked_error;
 #endif
 	if (fp->f_flag & FREAD && events & (POLLIN | POLLRDNORM))
-		if ((rpipe->pipe_state & PIPE_DIRECTW) ||
-		    (rpipe->pipe_buffer.cnt > 0))
+		if (rpipe->pipe_map.cnt > 0 || rpipe->pipe_buffer.cnt > 0)
 			revents |= events & (POLLIN | POLLRDNORM);
 
 	if (fp->f_flag & FWRITE && events & (POLLOUT | POLLWRNORM))
 		if (wpipe->pipe_present != PIPE_ACTIVE ||
 		    (wpipe->pipe_state & PIPE_EOF) ||
-		    (((wpipe->pipe_state & PIPE_DIRECTW) == 0) &&
+		    ((wpipe->pipe_state & PIPE_DIRECTW) == 0 &&
 		     ((wpipe->pipe_buffer.size - wpipe->pipe_buffer.cnt) >= PIPE_BUF ||
 			 wpipe->pipe_buffer.size == 0)))
 			revents |= events & (POLLOUT | POLLWRNORM);
@@ -1485,7 +1481,7 @@ pipe_stat(struct file *fp, struct stat *ub, struct ucred *active_cred,
 	bzero(ub, sizeof(*ub));
 	ub->st_mode = S_IFIFO;
 	ub->st_blksize = PAGE_SIZE;
-	if (pipe->pipe_state & PIPE_DIRECTW)
+	if (pipe->pipe_map.cnt != 0)
 		ub->st_size = pipe->pipe_map.cnt;
 	else
 		ub->st_size = pipe->pipe_buffer.cnt;
@@ -1727,7 +1723,7 @@ filt_piperead(struct knote *kn, long hint)
 
 	PIPE_LOCK_ASSERT(rpipe, MA_OWNED);
 	kn->kn_data = rpipe->pipe_buffer.cnt;
-	if ((kn->kn_data == 0) && (rpipe->pipe_state & PIPE_DIRECTW))
+	if (kn->kn_data == 0)
 		kn->kn_data = rpipe->pipe_map.cnt;
 
 	if ((rpipe->pipe_state & PIPE_EOF) ||