fuse: make args->in_args[0] to be always the header

This change sets up FUSE operations to always have headers in
args.in_args[0], even for opcodes without an actual header.
This step prepares for a clean separation of payload from headers,
initially it is used by fuse-over-io-uring.

For opcodes without a header, we use a zero-sized struct as a
placeholder. This approach:
- Keeps things consistent across all FUSE operations
- Will help with payload alignment later
- Avoids future issues when header sizes change

Op codes that already have an op code specific header do not
need modification.
Op codes that have neither payload nor op code headers
are not modified either (FUSE_READLINK and FUSE_DESTROY).
FUSE_BATCH_FORGET already has the header in the right place,
but is not using fuse_copy_args - as -over-uring is currently
not handling forgets it does not matter for now, but header
separation will later need special attention for that op code.

Correct the struct fuse_args->in_args array max size.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 74744c6f2860..5666900bee5e 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -310,7 +310,7 @@ struct fuse_args {
 	bool is_ext:1;
 	bool is_pinned:1;
 	bool invalidate_vmap:1;
-	struct fuse_in_arg in_args[3];
+	struct fuse_in_arg in_args[4];
 	struct fuse_arg out_args[2];
 	void (*end)(struct fuse_mount *fm, struct fuse_args *args, int error);
 	/* Used for kvec iter backed by vmalloc address */
@@ -947,6 +947,19 @@ struct fuse_mount {
 	struct rcu_head rcu;
 };
 
+/*
+ * Empty header for FUSE opcodes without specific header needs.
+ * Used as a placeholder in args->in_args[0] for consistency
+ * across all FUSE operations, simplifying request handling.
+ */
+struct fuse_zero_header {};
+
+static inline void fuse_set_zero_arg0(struct fuse_args *args)
+{
+	args->in_args[0].size = sizeof(struct fuse_zero_header);
+	args->in_args[0].value = NULL;
+}
+
 static inline struct fuse_mount *get_fuse_mount_super(struct super_block *sb)
 {
 	return sb->s_fs_info;