nfsd: Cap case-folding probe cost across READDIR entries

nfsd: Cap case-folding probe cost across READDIR entries

NFSv4 READDIR carries a per-entry attrmask. When the attrmask
includes FATTR4_CASE_INSENSITIVE or FATTR4_CASE_PRESERVING,
nfsd4_encode_fattr4() resolves each non-directory child's case
attributes by calling nfsd_get_case_info(), which dget_parent()s
back to the directory being read and re-runs the cred swap and LSM
probe per child. The encoder amplifies a single answer into one
prepare_kernel_cred() allocation, two LSM hooks, and one put_cred()
RCU callback for every non-directory entry.

No mainstream NFSv4 client has been observed to populate a READDIR
attrmask with these attributes; the Linux client queries them only
via SERVER_CAPS at mount time. The exposure is therefore to test
clients exploring corner cases and to hostile clients that submit
an attrmask designed to multiply server work by rd_dircount.

Probe the directory being read once and cache the result on
struct nfsd4_readdir for use by every non-directory child. The
probe targets the readdir filehandle's dentry, which is held for
the duration of the request, rather than dget_parent() of a
child's locklessly-acquired dentry; the latter could be moved out
of the directory by a concurrent rename and report attributes
from an unrelated parent. Directory entries continue to be
queried individually, because casefold-capable filesystems (ext4,
f2fs) report case state per directory. The other callers of
nfsd4_encode_fattr4() (single GETATTR, the buffer wrapper) pass
NULL for the cache pointer and behave as before.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260507-case-sensitivity-v14-0-e62cc8200435@oracle.com?part=14
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260515153515.362266-8-cel@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>