[PATCH v2 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.  The VFS checks the request before sending it,
so an existing name can also fail with the error of such a check, such
as -EROFS on a read-only mount or -EACCES without write permission on
the directory.  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          |  6 ++++++
 fs/fuse/inode.c           |  4 +++-
 include/uapi/linux/fuse.h | 12 +++++++++++-
 4 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 28516a1760e24..c4d47ef1a195c 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 c91af283aef20..1d4cc77887e50 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -537,6 +537,12 @@ struct fuse_conn {
 	 */
 	unsigned atomic_o_trunc:1;
 
+	/**
+	 * @excl_create: Create requests check for an existing name.
+	 * Only set in INIT
+	 */
+	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 1341b681d0ed3..97ff1b87186d9 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1445,6 +1445,8 @@ 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;
@@ -1503,7 +1505,7 @@ 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 616430a2973c6..0731e555ad9cb 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -257,6 +257,9 @@
  *  - add backing_id_64 to fuse_open_out
  *  - add FUSE_NOTIFY_BACKING_MAP, fuse_notify_backing_map_out, fuse_extent
  *  - add FUSE_BACKING_MAP_CREATE, FUSE_BACKING_MAP_CYCLIC
+ *
+ *  7.48
+ *  - add FUSE_EXCL_CREATE
  */
 
 #ifndef _LINUX_FUSE_H
@@ -292,7 +295,7 @@
 #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
@@ -480,6 +483,12 @@ struct fuse_file_lock {
  *		privilege that mounting virtiofs or fuseblk requires).
  *		Insufficiently privileged servers ignore it.
  * FUSE_PASSTHROUGH_V2: use 64 bit server allocated backing ID
+ * 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. The VFS checks the request before sending it, so an
+ *		existing name can also fail with the error of such a check,
+ *		such as -EROFS
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -530,6 +539,7 @@ struct fuse_file_lock {
 #define FUSE_HAS_IO_URING_BUFPOOL (1ULL << 43)
 #define FUSE_HAS_SYNCFS		(1ULL << 44)
 #define FUSE_PASSTHROUGH_V2	(1ULL << 45)
+#define FUSE_EXCL_CREATE	(1ULL << 46)
 
 /*
  * CUSE INIT request/reply flags
-- 
2.50.1