[PATCH 1/2] fuse: skip LOOKUP before exclusive create

[Jimmy Zuber <jamz@amazon.com>]

mkdir(), mknod(), symlink(), link() and renameat2() with RENAME_NOREPLACE
look the new name up with FUSE_LOOKUP before sending the request, which
costs a round trip to the server.

Add an INIT flag, FUSE_EXCL_CREATE, with which the server fails
FUSE_MKNOD, FUSE_MKDIR, FUSE_SYMLINK, FUSE_LINK, and FUSE_RENAME2 with
RENAME_NOREPLACE, with -EEXIST for an existing name.  When the server
sets it, the lookup of a name that is about to be created returns
without sending FUSE_LOOKUP, and an existing name is reported by the
-EEXIST of the request.  After a failed request the name stays
uncached, so the next lookup goes to the server.

A rename that may replace its target looks the target up, so that the
kernel can check the permission to replace it.

Assisted-by: Claude:claude-opus-5-5 [Claude-Code]
Signed-off-by: Jimmy Zuber <jamz@amazon.com>
---
 fs/fuse/dir.c             | 10 ++++++++++
 fs/fuse/fuse_i.h          |  3 +++
 fs/fuse/inode.c           |  4 +++-
 include/uapi/linux/fuse.h | 10 +++++++++-
 4 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index f48fafccce4b..e491e6b3215a 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -627,6 +627,16 @@ static struct dentry *fuse_lookup(struct inode *dir, struct dentry *entry,
 		return ERR_PTR(-EIO);
 
 	fc = get_fuse_conn_super(dir->i_sb);
+
+	/* The request fails with -EEXIST if the name exists */
+	if (fc->excl_create &&
+	    (flags & (LOOKUP_CREATE | LOOKUP_EXCL)) ==
+	    (LOOKUP_CREATE | LOOKUP_EXCL)) {
+		if (entry->d_name.len > fc->name_max)
+			return ERR_PTR(-ENAMETOOLONG);
+		return NULL;
+	}
+
 	epoch = atomic_read(&fc->epoch);
 
 	locked = fuse_lock_inode(dir);
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 8546855386b5..babbacdfb757 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -509,6 +509,9 @@ struct fuse_conn {
 	 */
 	unsigned atomic_o_trunc:1;
 
+	/** @excl_create: Create requests check for an existing name */
+	unsigned excl_create:1;
+
 	/**
 	 * @export_support: Filesystem supports NFS exporting.
 	 * Only set in INIT
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index cbb10e19e7e8..1f1be0c521a1 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1432,6 +1432,8 @@ static void process_init_reply(struct fuse_args *args, int error)
 
 			if (fuse_syncfs_enable(fc, flags))
 				fc->sync_fs = 1;
+			if (flags & FUSE_EXCL_CREATE)
+				fc->excl_create = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1490,7 +1492,7 @@ 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_EXCL_CREATE;
 #ifdef CONFIG_FUSE_VDAX
 	if (fm->fc->vdax)
 		flags |= FUSE_MAP_ALIGNMENT;
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 10a7f31c4bdf..8c079d3080bf 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -251,6 +251,9 @@
  *
  *  7.47
  *  - add FUSE_HAS_SYNCFS opt-in flag for privileged userspace servers
+ *
+ *  7.48
+ *  - add FUSE_EXCL_CREATE
  */
 
 #ifndef _LINUX_FUSE_H
@@ -286,7 +289,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 47
+#define FUSE_KERNEL_MINOR_VERSION 48
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -473,6 +476,10 @@ struct fuse_file_lock {
  *		with CAP_SYS_ADMIN in the initial user namespace (the same
  *		privilege that mounting virtiofs or fuseblk requires).
  *		Insufficiently privileged servers ignore it.
+ * FUSE_EXCL_CREATE: FUSE_MKNOD, FUSE_MKDIR, FUSE_SYMLINK, FUSE_LINK and
+ *		FUSE_RENAME2 with RENAME_NOREPLACE fail with -EEXIST for an
+ *		existing name, so the kernel sends them without a LOOKUP of
+ *		the name
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -522,6 +529,7 @@ struct fuse_file_lock {
 #define FUSE_REQUEST_TIMEOUT	(1ULL << 42)
 #define FUSE_HAS_IO_URING_BUFPOOL (1ULL << 43)
 #define FUSE_HAS_SYNCFS		(1ULL << 44)
+#define FUSE_EXCL_CREATE	(1ULL << 45)
 
 /**
  * CUSE INIT request/reply flags
-- 
2.50.1