[RFC PATCH v1 1/3] fuse: initial infrastructure for FUSE_LOOKUP_HANDLE support

[Luis Henriques <luis@igalia.com>]

This patch adds the initial infrastructure to implement the
FUSE_LOOKUP_HANDLE operation.  It simply defines the new operation and the
extra fuse_init_out field to set the maximum handle size.

Signed-off-by: Luis Henriques <luis@igalia.com>
---
 fs/fuse/fuse_i.h          | 4 ++++
 fs/fuse/inode.c           | 9 ++++++++-
 include/uapi/linux/fuse.h | 8 +++++++-
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index c2f2a48156d6..f4e9747ed8c7 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -907,6 +907,10 @@ struct fuse_conn {
 	/* Is synchronous FUSE_INIT allowed? */
 	unsigned int sync_init:1;
 
+	/** Is LOOKUP_HANDLE implemented by fs? */
+	unsigned int lookup_handle:1;
+	unsigned int max_handle_sz;
+
 	/* Use io_uring for communication */
 	unsigned int io_uring;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index d1babf56f254..b9b094c1bc36 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1453,6 +1453,13 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 
 			if (flags & FUSE_REQUEST_TIMEOUT)
 				timeout = arg->request_timeout;
+
+			if ((flags & FUSE_HAS_LOOKUP_HANDLE) &&
+			    (arg->max_handle_sz > 0) &&
+			    (arg->max_handle_sz <= FUSE_MAX_HANDLE_SZ)) {
+				fc->lookup_handle = 1;
+				fc->max_handle_sz = arg->max_handle_sz;
+			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1503,7 +1510,7 @@ static struct fuse_init_args *fuse_new_init(struct fuse_mount *fm)
 		FUSE_SECURITY_CTX | FUSE_CREATE_SUPP_GROUP |
 		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP |
 		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_ALLOW_IDMAP |
-		FUSE_REQUEST_TIMEOUT;
+		FUSE_REQUEST_TIMEOUT | FUSE_LOOKUP_HANDLE;
 #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 c13e1f9a2f12..4acf71b407c9 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -495,6 +495,7 @@ struct fuse_file_lock {
 #define FUSE_ALLOW_IDMAP	(1ULL << 40)
 #define FUSE_OVER_IO_URING	(1ULL << 41)
 #define FUSE_REQUEST_TIMEOUT	(1ULL << 42)
+#define FUSE_HAS_LOOKUP_HANDLE	(1ULL << 43)
 
 /**
  * CUSE INIT request/reply flags
@@ -663,6 +664,7 @@ enum fuse_opcode {
 	FUSE_TMPFILE		= 51,
 	FUSE_STATX		= 52,
 	FUSE_COPY_FILE_RANGE_64	= 53,
+	FUSE_LOOKUP_HANDLE	= 54,
 
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,
@@ -908,6 +910,9 @@ struct fuse_init_in {
 	uint32_t	unused[11];
 };
 
+/* Same value as MAX_HANDLE_SZ */
+#define FUSE_MAX_HANDLE_SZ 128
+
 #define FUSE_COMPAT_INIT_OUT_SIZE 8
 #define FUSE_COMPAT_22_INIT_OUT_SIZE 24
 
@@ -925,7 +930,8 @@ struct fuse_init_out {
 	uint32_t	flags2;
 	uint32_t	max_stack_depth;
 	uint16_t	request_timeout;
-	uint16_t	unused[11];
+	uint16_t	max_handle_sz;
+	uint16_t	unused[10];
 };
 
 #define CUSE_INIT_INFO_MAX 4096