[PATCH v4 2/3] fs: name_to_handle_at() support for "explicit connectable" file handles

Amir Goldstein
2024-10-11T09:00:31Z

nfsd encodes "connectable" file handles for the subtree_check feature,
which can be resolved to an open file with a connected path.
So far, userspace nfs server could not make use of this functionality.

Introduce a new flag AT_HANDLE_CONNECTABLE to name_to_handle_at(2).
When used, the encoded file handle is "explicitly connectable".

The "explicitly connectable" file handle sets bits in the high 16bit of
the handle_type field, so open_by_handle_at(2) will know that it needs
to open a file with a connected path.

old kernels will now recognize the handle_type with high bits set,
so "explicitly connectable" file handles cannot be decoded by
open_by_handle_at(2) on old kernels.

The flag AT_HANDLE_CONNECTABLE is not allowed together with either
AT_HANDLE_FID or AT_EMPTY_PATH.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
fs/fhandle.c | 48 ++++++++++++++++++++++++++++++++++----
include/linux/exportfs.h | 2 ++
include/uapi/linux/fcntl.h | 1 +
3 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 218511f38cbb..8339a1041025 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -31,6 +31,14 @@ static long do_sys_name_to_handle(const struct path *path,
if (!exportfs_can_encode_fh(path->dentry->d_sb->s_export_op, fh_flags))
return -EOPNOTSUPP;

+ /*
+ * A request to encode a connectable handle for a disconnected dentry
+ * is unexpected since AT_EMPTY_PATH is not allowed.
+ */
+ if (fh_flags & EXPORT_FH_CONNECTABLE &&
+ WARN_ON(path->dentry->d_flags & DCACHE_DISCONNECTED))
+ return -EINVAL;
+
if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle)))
return -EFAULT;

@@ -45,7 +53,7 @@ static long do_sys_name_to_handle(const struct path *path,
/* convert handle size to multiple of sizeof(u32) */
handle_dwords = f_handle.handle_bytes >> 2;

- /* we ask for a non connectable maybe decodeable file handle */
+ /* Encode a possibly decodeable/connectable file handle */
retval = exportfs_encode_fh(path->dentry,
(struct fid *)handle->f_handle,
&handle_dwords, fh_flags);
@@ -67,8 +75,23 @@ static long do_sys_name_to_handle(const struct path *path,
* non variable part of the file_handle
*/
handle_bytes = 0;
- } else
+ } else {
+ /*
+ * When asked to encode a connectable file handle, encode this
+ * property in the file handle itself, so that we later know
+ * how to decode it.
+ * For sanity, also encode in the file handle if the encoded
+ * object is a directory and verify this during decode, because
+ * decoding directory file handles is quite different than
+ * decoding connectable non-directory file handles.
+ */
+ if (fh_flags & EXPORT_FH_CONNECTABLE) {
+ handle->handle_type |= FILEID_IS_CONNECTABLE;
+ if (d_is_dir(path->dentry))
+ fh_flags |= FILEID_IS_DIR;
+ }
retval = 0;
+ }
/* copy the mount id */
if (unique_mntid) {
if (put_user(real_mount(path->mnt)->mnt_id_unique,
@@ -109,15 +132,30 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
{
struct path path;
int lookup_flags;
- int fh_flags;
+ int fh_flags = 0;
int err;

if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID |
- AT_HANDLE_MNT_ID_UNIQUE))
+ AT_HANDLE_MNT_ID_UNIQUE | AT_HANDLE_CONNECTABLE))
+ return -EINVAL;
+
+ /*
+ * AT_HANDLE_FID means there is no intention to decode file handle
+ * AT_HANDLE_CONNECTABLE means there is an intention to decode a
+ * connected fd (with known path), so these flags are conflicting.
+ * AT_EMPTY_PATH could be used along with a dfd that refers to a
+ * disconnected non-directory, which cannot be used to encode a
+ * connectable file handle, because its parent is unknown.
+ */
+ if (flag & AT_HANDLE_CONNECTABLE &&
+ flag & (AT_HANDLE_FID | AT_EMPTY_PATH))
return -EINVAL;
+ else if (flag & AT_HANDLE_FID)
+ fh_flags |= EXPORT_FH_FID;
+ else if (flag & AT_HANDLE_CONNECTABLE)
+ fh_flags |= EXPORT_FH_CONNECTABLE;

lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
- fh_flags = (flag & AT_HANDLE_FID) ? EXPORT_FH_FID : 0;
if (flag & AT_EMPTY_PATH)
lookup_flags |= LOOKUP_EMPTY;
err = user_path_at(dfd, name, lookup_flags, &path);
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 5e14d4500a75..4ee42b2cf4ab 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -169,6 +169,8 @@ struct fid {
#define FILEID_USER_FLAGS(type) ((type) & FILEID_USER_FLAGS_MASK)

/* Flags supported in encoded handle_type that is exported to user */
+#define FILEID_IS_CONNECTABLE 0x10000
+#define FILEID_IS_DIR 0x20000
#define FILEID_VALID_USER_FLAGS (0)

/**
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 87e2dec79fea..56ff2100e021 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -153,6 +153,7 @@
object identity and may not be
usable with open_by_handle_at(2). */
#define AT_HANDLE_MNT_ID_UNIQUE 0x001 /* Return the u64 unique mount ID. */
+#define AT_HANDLE_CONNECTABLE 0x002 /* Request a connectable file handle */

#if defined(__KERNEL__)
#define AT_GETATTR_NOSEC 0x80000000
--
2.34.1