Fix errant EFAULT during writes (#8719)

Commit 98bb45e resolved a deadlock which could occur when
handling a page fault in zfs_write().  This change added
the uio_fault_disable field to the uio structure but failed
to initialize it to B_FALSE.  This uninitialized field would
cause uiomove_iov() to call __copy_from_user_inatomic()
instead of copy_from_user() resulting in unexpected EFAULTs.

Resolve the issue by fully initializing the uio, and clearing
the uio_fault_disable flags after it's used in zfs_write().

Additionally, reorder the uio_t field assignments to match
the order the fields are declared in the  structure.

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8640 
Closes #8719

module/zfs/zfs_vnops.c

@@ -822,6 +822,7 @@ zfs_write(struct inode *ip, uio_t *uio, int ioflag, cred_t *cr)
 			uio->uio_fault_disable = B_TRUE;
 			error = dmu_write_uio_dbuf(sa_get_db(zp->z_sa_hdl),
 			    uio, nbytes, tx);
+			uio->uio_fault_disable = B_FALSE;
 			if (error == EFAULT) {
 				dmu_tx_commit(tx);
 				if (uio_prefaultpages(MIN(n, max_blksz), uio)) {

module/zfs/zpl_file.c

@@ -246,17 +246,17 @@ zpl_read_common_iovec(struct inode *ip, const struct iovec *iovp, size_t count,
     cred_t *cr, size_t skip)
 {
 	ssize_t read;
-	uio_t uio;
+	uio_t uio = { { 0 }, 0 };
 	int error;
 	fstrans_cookie_t cookie;
 
 	uio.uio_iov = iovp;
-	uio.uio_skip = skip;
-	uio.uio_resid = count;
 	uio.uio_iovcnt = nr_segs;
 	uio.uio_loffset = *ppos;
-	uio.uio_limit = MAXOFFSET_T;
 	uio.uio_segflg = segment;
+	uio.uio_limit = MAXOFFSET_T;
+	uio.uio_resid = count;
+	uio.uio_skip = skip;
 
 	cookie = spl_fstrans_mark();
 	error = -zfs_read(ip, &uio, flags, cr);
@@ -356,7 +356,7 @@ zpl_write_common_iovec(struct inode *ip, const struct iovec *iovp, size_t count,
     cred_t *cr, size_t skip)
 {
 	ssize_t wrote;
-	uio_t uio;
+	uio_t uio = { { 0 }, 0 };
 	int error;
 	fstrans_cookie_t cookie;
 
@@ -364,12 +364,12 @@ zpl_write_common_iovec(struct inode *ip, const struct iovec *iovp, size_t count,
 		*ppos = i_size_read(ip);
 
 	uio.uio_iov = iovp;
-	uio.uio_skip = skip;
-	uio.uio_resid = count;
 	uio.uio_iovcnt = nr_segs;
 	uio.uio_loffset = *ppos;
-	uio.uio_limit = MAXOFFSET_T;
 	uio.uio_segflg = segment;
+	uio.uio_limit = MAXOFFSET_T;
+	uio.uio_resid = count;
+	uio.uio_skip = skip;
 
 	cookie = spl_fstrans_mark();
 	error = -zfs_write(ip, &uio, flags, cr);

module/zfs/zpl_inode.c

@@ -493,7 +493,7 @@ zpl_get_link_common(struct dentry *dentry, struct inode *ip, char **link)
 	fstrans_cookie_t cookie;
 	cred_t *cr = CRED();
 	struct iovec iov;
-	uio_t uio;
+	uio_t uio = { { 0 }, 0 };
 	int error;
 
 	crhold(cr);
@@ -503,9 +503,8 @@ zpl_get_link_common(struct dentry *dentry, struct inode *ip, char **link)
 
 	uio.uio_iov = &iov;
 	uio.uio_iovcnt = 1;
-	uio.uio_skip = 0;
-	uio.uio_resid = (MAXPATHLEN - 1);
 	uio.uio_segflg = UIO_SYSSPACE;
+	uio.uio_resid = (MAXPATHLEN - 1);
 
 	cookie = spl_fstrans_mark();
 	error = -zfs_readlink(ip, &uio, cr);

module/zfs/zvol.c

@@ -727,12 +727,12 @@ static void
 uio_from_bio(uio_t *uio, struct bio *bio)
 {
 	uio->uio_bvec = &bio->bi_io_vec[BIO_BI_IDX(bio)];
-	uio->uio_skip = BIO_BI_SKIP(bio);
-	uio->uio_resid = BIO_BI_SIZE(bio);
 	uio->uio_iovcnt = bio->bi_vcnt - BIO_BI_IDX(bio);
 	uio->uio_loffset = BIO_BI_SECTOR(bio) << 9;
-	uio->uio_limit = MAXOFFSET_T;
 	uio->uio_segflg = UIO_BVEC;
+	uio->uio_limit = MAXOFFSET_T;
+	uio->uio_resid = BIO_BI_SIZE(bio);
+	uio->uio_skip = BIO_BI_SKIP(bio);
 }
 
 static void
@@ -742,7 +742,7 @@ zvol_write(void *arg)
 
 	zv_request_t *zvr = arg;
 	struct bio *bio = zvr->bio;
-	uio_t uio;
+	uio_t uio = { { 0 }, 0 };
 	uio_from_bio(&uio, bio);
 
 	zvol_state_t *zv = zvr->zv;
@@ -897,7 +897,7 @@ zvol_read(void *arg)
 
 	zv_request_t *zvr = arg;
 	struct bio *bio = zvr->bio;
-	uio_t uio;
+	uio_t uio = { { 0 }, 0 };
 	uio_from_bio(&uio, bio);
 
 	zvol_state_t *zv = zvr->zv;