[PATCH RFC] fuse: require FUSE drivers to opt-in for local file leases

[Jeff Layton]

Traditionally, we've allowed people to set leases on FUSE inodes.  Some
FUSE drivers are effectively local filesystems and should be fine with
kernel-internal lease support. But others are backed by a network server
that may have multiple clients, or may be backed by something non-file
like entirely.

Have the filesytem driver to set a fuse_conn flag to indicate whether it
wants support for local, in-kernel leases. If the flag is unset (the
default), then setlease attempts will fail with -EINVAL, indicating that
leases aren't supported on that inode.

Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
This is only tested for compilation, but it's fairly straightforward.
Having the FUSE drivers opt-out of this support might be more
backward-compatible, but that is a bit more dangerous. I'd rather driver
maintainer consciously opt-in.
---
 fs/fuse/file.c            | 11 +++++++++++
 fs/fuse/fuse_i.h          |  3 +++
 fs/fuse/inode.c           |  4 +++-
 include/uapi/linux/fuse.h |  1 +
 4 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index a56e7bffd000..3d9aef376783 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -3298,6 +3298,16 @@ static ssize_t fuse_copy_file_range(struct file *src_file, loff_t src_off,
 	return ret;
 }
 
+static int fuse_setlease(struct file *file, int arg,
+			 struct file_lease **flp, void **priv)
+{
+	struct fuse_conn *fc = get_fuse_conn(file_inode(file));
+
+	if (fc->local_leases)
+		return generic_setlease(file, arg, flp, priv);
+	return -EINVAL;
+}
+
 static const struct file_operations fuse_file_operations = {
 	.llseek		= fuse_file_llseek,
 	.read_iter	= fuse_file_read_iter,
@@ -3317,6 +3327,7 @@ static const struct file_operations fuse_file_operations = {
 	.poll		= fuse_file_poll,
 	.fallocate	= fuse_file_fallocate,
 	.copy_file_range = fuse_copy_file_range,
+	.setlease	= fuse_setlease,
 };
 
 static const struct address_space_operations fuse_file_aops  = {
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index b24084b60864..2909788bf69f 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -860,6 +860,9 @@ struct fuse_conn {
 	/** Passthrough support for read/write IO */
 	unsigned int passthrough:1;
 
+	/** Opt-in to local kernel lease support */
+	unsigned int local_leases:1;
+
 	/** Maximum stack depth for passthrough backing files */
 	int max_stack_depth;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 3a5d88878335..6958c7690e68 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1330,6 +1330,8 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 			}
 			if (flags & FUSE_NO_EXPORT_SUPPORT)
 				fm->sb->s_export_op = &fuse_export_fid_operations;
+			if (flags & FUSE_LOCAL_LEASES)
+				fc->local_leases = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1377,7 +1379,7 @@ void fuse_send_init(struct fuse_mount *fm)
 		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
 		FUSE_SECURITY_CTX | FUSE_CREATE_SUPP_GROUP |
 		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP |
-		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND;
+		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_LOCAL_LEASES;
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		flags |= FUSE_MAP_ALIGNMENT;
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index d08b99d60f6f..76322808159d 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -463,6 +463,7 @@ struct fuse_file_lock {
 #define FUSE_PASSTHROUGH	(1ULL << 37)
 #define FUSE_NO_EXPORT_SUPPORT	(1ULL << 38)
 #define FUSE_HAS_RESEND		(1ULL << 39)
+#define FUSE_LOCAL_LEASES	(1ULL << 40)
 
 /* Obsolete alias for FUSE_DIRECT_IO_ALLOW_MMAP */
 #define FUSE_DIRECT_IO_RELAX	FUSE_DIRECT_IO_ALLOW_MMAP

---
base-commit: 0a7b0acecea273c8816f4f5b0e189989470404cf
change-id: 20240319-setlease-ce31fb8777b0

Best regards,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>