fs: add open_tree_attr()

Add open_tree_attr() which allow to atomically create a detached mount
tree and set mount options on it. If OPEN_TREE_CLONE is used this will
allow the creation of a detached mount with a new set of mount options
without it ever being exposed to userspace without that set of mount
options applied.

Link: https://lore.kernel.org/r/20250128-work-mnt_idmap-update-v2-v1-3-c25feb0d2eb3@kernel.org
Reviewed-by: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

arch/alpha/kernel/syscalls/syscall.tbl

@@ -506,3 +506,4 @@
 574	common	getxattrat			sys_getxattrat
 575	common	listxattrat			sys_listxattrat
 576	common	removexattrat			sys_removexattrat
+577	common	open_tree_attr			sys_open_tree_attr

arch/arm/tools/syscall.tbl

@@ -481,3 +481,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/arm64/tools/syscall_32.tbl

@@ -478,3 +478,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/m68k/kernel/syscalls/syscall.tbl

@@ -466,3 +466,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/microblaze/kernel/syscalls/syscall.tbl

@@ -472,3 +472,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/mips/kernel/syscalls/syscall_n32.tbl

@@ -405,3 +405,4 @@
 464	n32	getxattrat			sys_getxattrat
 465	n32	listxattrat			sys_listxattrat
 466	n32	removexattrat			sys_removexattrat
+467	n32	open_tree_attr			sys_open_tree_attr

arch/mips/kernel/syscalls/syscall_n64.tbl

@@ -381,3 +381,4 @@
 464	n64	getxattrat			sys_getxattrat
 465	n64	listxattrat			sys_listxattrat
 466	n64	removexattrat			sys_removexattrat
+467	n64	open_tree_attr			sys_open_tree_attr

arch/mips/kernel/syscalls/syscall_o32.tbl

@@ -454,3 +454,4 @@
 464	o32	getxattrat			sys_getxattrat
 465	o32	listxattrat			sys_listxattrat
 466	o32	removexattrat			sys_removexattrat
+467	o32	open_tree_attr			sys_open_tree_attr

arch/parisc/kernel/syscalls/syscall.tbl

@@ -465,3 +465,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/powerpc/kernel/syscalls/syscall.tbl

@@ -557,3 +557,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/s390/kernel/syscalls/syscall.tbl

@@ -469,3 +469,4 @@
 464  common	getxattrat		sys_getxattrat			sys_getxattrat
 465  common	listxattrat		sys_listxattrat			sys_listxattrat
 466  common	removexattrat		sys_removexattrat		sys_removexattrat
+467  common	open_tree_attr		sys_open_tree_attr		sys_open_tree_attr

arch/sh/kernel/syscalls/syscall.tbl

@@ -470,3 +470,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/sparc/kernel/syscalls/syscall.tbl

@@ -512,3 +512,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

arch/x86/entry/syscalls/syscall_32.tbl

@@ -472,3 +472,4 @@
 464	i386	getxattrat		sys_getxattrat
 465	i386	listxattrat		sys_listxattrat
 466	i386	removexattrat		sys_removexattrat
+467	i386	open_tree_attr		sys_open_tree_attr

arch/x86/entry/syscalls/syscall_64.tbl

@@ -390,6 +390,7 @@
 464	common	getxattrat		sys_getxattrat
 465	common	listxattrat		sys_listxattrat
 466	common	removexattrat		sys_removexattrat
+467	common	open_tree_attr		sys_open_tree_attr
 
 #
 # Due to a historical design error, certain syscalls are numbered differently

arch/xtensa/kernel/syscalls/syscall.tbl

@@ -437,3 +437,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr

fs/namespace.c

@@ -4995,6 +4995,45 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
 	return err;
 }
 
+SYSCALL_DEFINE5(open_tree_attr, int, dfd, const char __user *, filename,
+		unsigned, flags, struct mount_attr __user *, uattr,
+		size_t, usize)
+{
+	struct file __free(fput) *file = NULL;
+	int fd;
+
+	if (!uattr && usize)
+		return -EINVAL;
+
+	file = vfs_open_tree(dfd, filename, flags);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+
+	if (uattr) {
+		int ret;
+		struct mount_kattr kattr = {
+			.recurse = !!(flags & AT_RECURSIVE),
+		};
+
+		ret = copy_mount_setattr(uattr, usize, &kattr);
+		if (ret)
+			return ret;
+
+		ret = do_mount_setattr(&file->f_path, &kattr);
+		if (ret)
+			return ret;
+
+		finish_mount_kattr(&kattr);
+	}
+
+	fd = get_unused_fd_flags(flags & O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	fd_install(fd, no_free_ptr(file));
+	return fd;
+}
+
 int show_path(struct seq_file *m, struct dentry *root)
 {
 	if (root->d_sb->s_op->show_path)

include/linux/syscalls.h

@@ -951,6 +951,10 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
 asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
 			 int flags, uint32_t sig);
 asmlinkage long sys_open_tree(int dfd, const char __user *path, unsigned flags);
+asmlinkage long sys_open_tree_attr(int dfd, const char __user *path,
+				   unsigned flags,
+				   struct mount_attr __user *uattr,
+				   size_t usize);
 asmlinkage long sys_move_mount(int from_dfd, const char __user *from_path,
 			       int to_dfd, const char __user *to_path,
 			       unsigned int ms_flags);

include/uapi/asm-generic/unistd.h

@@ -849,9 +849,11 @@ __SYSCALL(__NR_getxattrat, sys_getxattrat)
 __SYSCALL(__NR_listxattrat, sys_listxattrat)
 #define __NR_removexattrat 466
 __SYSCALL(__NR_removexattrat, sys_removexattrat)
+#define __NR_open_tree_attr 467
+__SYSCALL(__NR_open_tree_attr, sys_open_tree_attr)
 
 #undef __NR_syscalls
-#define __NR_syscalls 467
+#define __NR_syscalls 468
 
 /*
  * 32 bit systems traditionally used different

scripts/syscall.tbl

@@ -407,3 +407,4 @@
 464	common	getxattrat			sys_getxattrat
 465	common	listxattrat			sys_listxattrat
 466	common	removexattrat			sys_removexattrat
+467	common	open_tree_attr			sys_open_tree_attr