Merge patch series "API for exporting connectable file handles to userspace"

Amir Goldstein <amir73il@gmail.com> says:

These patches bring the NFS connectable file handles feature to
userspace servers.

They rely on Christian's and Aleksa's changes recently merged to v6.12.

The API I chose for encoding conenctable file handles is pretty
conventional (AT_HANDLE_CONNECTABLE).

open_by_handle_at(2) does not have AT_ flags argument, but also, I find
it more useful API that encoding a connectable file handle can mandate
the resolving of a connected fd, without having to opt-in for a
connected fd independently.

I chose to implemnent this by using upper bits in the handle type field
It may be that out-of-tree filesystems return a handle type with upper
bits set, but AFAIK, no in-tree filesystem does that.
I added some warnings just in case we encouter that.

I have written an fstest [1] and a man page draft [2] for the feature.

[1] https://github.com/amir73il/xfstests/commits/connectable-fh/
[2] https://github.com/amir73il/man-pages/commits/connectable-fh/

* patches from https://lore.kernel.org/r/20241011090023.655623-1-amir73il@gmail.com:
  fs: open_by_handle_at() support for decoding "explicit connectable" file handles
  fs: name_to_handle_at() support for "explicit connectable" file handles
  fs: prepare for "explicit connectable" file handles

Link: https://lore.kernel.org/r/20241011090023.655623-1-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

4 files changed, 98 insertions, 8 deletions

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 4f2dd4ab4486..0c899cfba578 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -382,14 +382,24 @@ int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
 			     int *max_len, struct inode *parent, int flags)
 {
 	const struct export_operations *nop = inode->i_sb->s_export_op;
+	enum fid_type type;
 
 	if (!exportfs_can_encode_fh(nop, flags))
 		return -EOPNOTSUPP;
 
 	if (!nop && (flags & EXPORT_FH_FID))
-		return exportfs_encode_ino64_fid(inode, fid, max_len);
+		type = exportfs_encode_ino64_fid(inode, fid, max_len);
+	else
+		type = nop->encode_fh(inode, fid->raw, max_len, parent);
+
+	if (type > 0 && FILEID_USER_FLAGS(type)) {
+		pr_warn_once("%s: unexpected fh type value 0x%x from fstype %s.\n",
+			     __func__, type, inode->i_sb->s_type->name);
+		return -EINVAL;
+	}
+
+	return type;
 
-	return nop->encode_fh(inode, fid->raw, max_len, parent);
 }
 EXPORT_SYMBOL_GPL(exportfs_encode_inode_fh);
 
@@ -436,6 +446,9 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
 	char nbuf[NAME_MAX+1];
 	int err;
 
+	if (fileid_type < 0 || FILEID_USER_FLAGS(fileid_type))
+		return ERR_PTR(-EINVAL);
+
 	/*
 	 * Try to get any dentry for the given file handle from the filesystem.
 	 */
diff --git a/fs/fhandle.c b/fs/fhandle.c
index 82df28d45cd7..75cfd190cd69 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);
@@ -208,7 +246,13 @@ static int vfs_dentry_acceptable(void *context, struct dentry *dentry)
 
 	if (!(ctx->flags & HANDLE_CHECK_SUBTREE) || d == root)
 		retval = 1;
-	WARN_ON_ONCE(d != root && d != root->d_sb->s_root);
+	/*
+	 * exportfs_decode_fh_raw() does not call acceptable() callback with
+	 * a disconnected directory dentry, so we should have reached either
+	 * mount fd directory or sb root.
+	 */
+	if (ctx->fh_flags & EXPORT_FH_DIR_ONLY)
+		WARN_ON_ONCE(d != root && d != root->d_sb->s_root);
 	dput(d);
 	return retval;
 }
@@ -307,6 +351,12 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		retval = -EINVAL;
 		goto out_path;
 	}
+	if (f_handle.handle_type < 0 ||
+	    FILEID_USER_FLAGS(f_handle.handle_type) & ~FILEID_VALID_USER_FLAGS) {
+		retval = -EINVAL;
+		goto out_path;
+	}
+
 	handle = kmalloc(struct_size(handle, f_handle, f_handle.handle_bytes),
 			 GFP_KERNEL);
 	if (!handle) {
@@ -322,6 +372,19 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		goto out_handle;
 	}
 
+	/*
+	 * If handle was encoded with AT_HANDLE_CONNECTABLE, verify that we
+	 * are decoding an fd with connected path, which is accessible from
+	 * the mount fd path.
+	 */
+	if (f_handle.handle_type & FILEID_IS_CONNECTABLE) {
+		ctx.fh_flags |= EXPORT_FH_CONNECTABLE;
+		ctx.flags |= HANDLE_CHECK_SUBTREE;
+	}
+	if (f_handle.handle_type & FILEID_IS_DIR)
+		ctx.fh_flags |= EXPORT_FH_DIR_ONLY;
+	/* Filesystem code should not be exposed to user flags */
+	handle->handle_type &= ~FILEID_USER_FLAGS_MASK;
 	retval = do_handle_to_path(handle, path, &ctx);
 
 out_handle:
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 893a1d21dc1c..fcab6ab1d38a 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -160,6 +160,19 @@ struct fid {
 #define EXPORT_FH_FID		0x2 /* File handle may be non-decodeable */
 #define EXPORT_FH_DIR_ONLY	0x4 /* Only decode file handle for a directory */
 
+/*
+ * Filesystems use only lower 8 bits of file_handle type for fid_type.
+ * name_to_handle_at() uses upper 16 bits of type as user flags to be
+ * interpreted by open_by_handle_at().
+ */
+#define FILEID_USER_FLAGS_MASK	0xffff0000
+#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	(FILEID_IS_CONNECTABLE | FILEID_IS_DIR)
+
 /**
  * struct export_operations - for nfsd to communicate with file systems
  * @encode_fh:      encode a file handle fragment from a dentry
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