bl_open_path() resolves pNFS block device paths under /dev/disk/by-id/
via bdev_file_open_by_path() -> lookup_bdev() -> kern_path(). This
path resolution uses current->fs->root.

With kthreads now starting in nullfs, this fails when the call
originates from writeback kworker context because current->fs->root
points at the empty nullfs. The full callchain from kworker is:

  wb_workfn                              [kworker writeback callback]
    ...
      nfs_writepages                     [address_space_operations.writepages]
        nfs_do_writepage
          nfs_pageio_add_request
            ...
              bl_pg_init_write           [nfs_pageio_ops.pg_init]
                pnfs_generic_pg_init_write
                  pnfs_update_layout
                    nfs4_proc_layoutget  [synchronous RPC]
                      pnfs_layout_process
                        bl_alloc_lseg
                          bl_alloc_extent
                            bl_find_get_deviceid
                              bl_alloc_deviceid_node
                                bl_parse_deviceid
                                  bl_parse_scsi
                                    bl_open_path
                                      bdev_file_open_by_path
                                        lookup_bdev
                                          kern_path  <- current->fs->root

bl_open_path() can also be reached from userspace process context (e.g.
open, read, write syscalls via pnfs_update_layout). In that case
current->fs must not be overridden as the path should resolve against
the calling process's filesystem root.

Add a tsk_is_kthread() conditional in bl_open_path() to only apply
scoped_with_init_fs() in kthread context.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/nfs/blocklayout/dev.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/blocklayout/dev.c b/fs/nfs/blocklayout/dev.c
index cc6327d97a91..eed960839608 100644
--- a/fs/nfs/blocklayout/dev.c
+++ b/fs/nfs/blocklayout/dev.c
@@ -4,6 +4,7 @@
  */
 #include <linux/sunrpc/svc.h>
 #include <linux/blkdev.h>
+#include <linux/fs_struct.h>
 #include <linux/nfs4.h>
 #include <linux/nfs_fs.h>
 #include <linux/nfs_xdr.h>
@@ -363,21 +364,27 @@ static struct file *
 bl_open_path(struct pnfs_block_volume *v, const char *prefix)
 {
 	struct file *bdev_file;
-	const char *devname;
+	const char *devname __free(kfree) = NULL;
 
 	devname = kasprintf(GFP_KERNEL, "/dev/disk/by-id/%s%*phN",
 			prefix, v->scsi.designator_len, v->scsi.designator);
 	if (!devname)
 		return ERR_PTR(-ENOMEM);
 
-	bdev_file = bdev_file_open_by_path(devname, BLK_OPEN_READ | BLK_OPEN_WRITE,
+	if (tsk_is_kthread(current)) {
+		scoped_with_init_fs()
+			bdev_file = bdev_file_open_by_path(devname,
+					BLK_OPEN_READ | BLK_OPEN_WRITE,
 					NULL, NULL);
+	} else {
+		bdev_file = bdev_file_open_by_path(devname,
+				BLK_OPEN_READ | BLK_OPEN_WRITE, NULL, NULL);
+	}
 	if (IS_ERR(bdev_file)) {
 		dprintk("failed to open device %s (%ld)\n",
 			devname, PTR_ERR(bdev_file));
 	}
 
-	kfree(devname);
 	return bdev_file;
 }
 

-- 
2.47.3