[PATCH v2] nfs: fix ENXIO on O_CREAT open of existing symlink over NFSv3

When open(2) is called with O_CREAT on a path that already exists as a
symlink, over an NFSv3 mount with a cold dcache, the kernel returns
ENXIO instead of following the symlink to its target.

Reproducer script (MNT is an NFSv3 mount, kernel is 7.1-rc6):

MNT=/mnt/export
ln -sf /tmp/target $MNT/mylink
echo 3 | sudo tee /proc/sys/vm/drop_caches   # cold dcache

python3 - <<'EOF'
import os
fd = os.open('/mnt/export/mylink', os.O_WRONLY | os.O_CREAT | os.O_APPEND, 0o666)
os.close(fd)
EOF

Expected: success (follow symlink, open target)
Actual:   OSError: [Errno 6] No such device or address

The bug does not trigger when the dcache is warm (e.g. after a prior
stat(2)), because lookup_open() then finds a positive dentry and skips
atomic_open entirely, leaving symlink resolution to the VFS.

Root cause:
nfs_atomic_open_v23(), registered as inode->i_op->atomic_open for
NFSv3, handles O_CREAT by sending a CREATE UNCHECKED RPC. As
implemented in nfsd3_create_file() (fs/nfsd/nfs3proc.c) and as required
by RFC 1813 (3.3.8), when the name already exists as a non-regular file
the server returns NFS3_OK with the existing object's file handle rather
than NFS3ERR_EXIST causing nfs_do_create() to return 0 with the
dentry now pointing to a symlink.
The code then unconditionally calls finish_open(), which dispatches
through inode->i_fop->open(). Symlink inodes never have i_fop set — the
VFS initialises it to &no_open_fops because POSIX requires open(2) to
follow symlinks, never open them directly. no_open() returns -ENXIO.

Fix:
After nfs_do_create() succeeds, verify the returned inode is a regular
file before calling finish_open(). If the object is not regular, return
finish_no_open(file, NULL) so the VFS follows the symlink through the
normal open path. NULL is passed because nfs_do_create() instantiates
the inode on the dentry already owned by the caller; passing dentry back
would cause atomic_open() to dput() it a second time.
!S_ISREG() is used rather than S_ISLNK() to cover any other non-regular
types a server might return.

Changes in v2:
- Pass NULL to finish_no_open() per Trond's feedback.

Fixes: 7c6c5249f061 ("NFS: add atomic_open for NFSv3 to handle O_TRUNC correctly.")
Link: https://lore.kernel.org/linux-nfs/20260614122911.3485467-1-michael.nemanov-8Du6NiZp2BlWk0Htik3J/[email protected]/ (v1)
Signed-off-by: Michael Nemanov <michael.nemanov-8Du6NiZp2BlWk0Htik3J/[email protected]>
Tested-by: Michael Nemanov <michael.nemanov-8Du6NiZp2BlWk0Htik3J/[email protected]>
---
 fs/nfs/dir.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index e9ce1883288c5..c3481d9c74334 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -2317,6 +2317,13 @@ int nfs_atomic_open_v23(struct inode *dir, struct dentry *dentry,
 	if (open_flags & O_CREAT) {
 		error = nfs_do_create(dir, dentry, mode, open_flags);
 		if (!error) {
+			/* With UNCHECKED mode, a server may return NFS3_OK for
+			 * a pre-existing non-regular file (e.g. a symlink).
+			 * Let the VFS handle it; calling finish_open() would
+			 * hit no_open() and return -ENXIO.
+			 */
+			if (d_inode(dentry) && !S_ISREG(d_inode(dentry)->i_mode))
+				return finish_no_open(file, NULL);
 			file->f_mode |= FMODE_CREATED;
 			return finish_open(file, dentry, NULL);
 		} else if (error != -EEXIST || open_flags & O_EXCL)
-- 
2.43.7