From 8f6116b5b77b0536d2ad7482ee42bfe58b8fac01 Mon Sep 17 00:00:00 2001
From: Jeff Layton <jlayton@kernel.org>
Date: Thu, 6 Feb 2025 07:51:52 -0500
Subject: [PATCH 1/6] statmount: add a new supported_mask field

Some of the fields in the statmount() reply can be optional. If the
kernel has nothing to emit in that field, then it doesn't set the flag
in the reply. This presents a problem: There is currently no way to
know what mask flags the kernel supports since you can't always count on
them being in the reply.

Add a new STATMOUNT_SUPPORTED_MASK flag and field that the kernel can
set in the reply. Userland can use this to determine if the fields it
requires from the kernel are supported. This also gives us a way to
deprecate fields in the future, if that should become necessary.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20250206-statmount-v2-1-6ae70a21c2ab@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c             | 23 +++++++++++++++++++++++
 include/uapi/linux/mount.h |  4 +++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index d470a369d42bc..c5ada826d4889 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5410,6 +5410,21 @@ static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
 	return 0;
 }
 
+/* This must be updated whenever a new flag is added */
+#define STATMOUNT_SUPPORTED (STATMOUNT_SB_BASIC | \
+			     STATMOUNT_MNT_BASIC | \
+			     STATMOUNT_PROPAGATE_FROM | \
+			     STATMOUNT_MNT_ROOT | \
+			     STATMOUNT_MNT_POINT | \
+			     STATMOUNT_FS_TYPE | \
+			     STATMOUNT_MNT_NS_ID | \
+			     STATMOUNT_MNT_OPTS | \
+			     STATMOUNT_FS_SUBTYPE | \
+			     STATMOUNT_SB_SOURCE | \
+			     STATMOUNT_OPT_ARRAY | \
+			     STATMOUNT_OPT_SEC_ARRAY | \
+			     STATMOUNT_SUPPORTED_MASK)
+
 static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 			struct mnt_namespace *ns)
 {
@@ -5479,9 +5494,17 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
 		statmount_mnt_ns_id(s, ns);
 
+	if (!err && s->mask & STATMOUNT_SUPPORTED_MASK) {
+		s->sm.mask |= STATMOUNT_SUPPORTED_MASK;
+		s->sm.supported_mask = STATMOUNT_SUPPORTED;
+	}
+
 	if (err)
 		return err;
 
+	/* Are there bits in the return mask not present in STATMOUNT_SUPPORTED? */
+	WARN_ON_ONCE(~STATMOUNT_SUPPORTED & s->sm.mask);
+
 	return 0;
 }
 
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index c07008816acae..c553dc4ba6840 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -179,7 +179,8 @@ struct statmount {
 	__u32 opt_array;	/* [str] Array of nul terminated fs options */
 	__u32 opt_sec_num;	/* Number of security options */
 	__u32 opt_sec_array;	/* [str] Array of nul terminated security options */
-	__u64 __spare2[46];
+	__u64 supported_mask;	/* Mask flags that this kernel supports */
+	__u64 __spare2[45];
 	char str[];		/* Variable size part containing strings */
 };
 
@@ -217,6 +218,7 @@ struct mnt_id_req {
 #define STATMOUNT_SB_SOURCE		0x00000200U	/* Want/got sb_source */
 #define STATMOUNT_OPT_ARRAY		0x00000400U	/* Want/got opt_... */
 #define STATMOUNT_OPT_SEC_ARRAY		0x00000800U	/* Want/got opt_sec... */
+#define STATMOUNT_SUPPORTED_MASK	0x00001000U	/* Want/got supported mask flags */
 
 /*
  * Special @mnt_id values that can be passed to listmount

From 901766df440f33b5aa30a491dc3e78655a627041 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Tue, 28 Jan 2025 11:33:39 +0100
Subject: [PATCH 2/6] fs: add vfs_open_tree() helper

Split out vfs_open_tree() from open_tree() so we can use it in later
patches.

Link: https://lore.kernel.org/r/20250128-work-mnt_idmap-update-v2-v1-1-c25feb0d2eb3@kernel.org
Reviewed-by: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c | 49 ++++++++++++++++++++++++++-----------------------
 1 file changed, 26 insertions(+), 23 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index c5ada826d4889..7604cdacba20b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3002,24 +3002,22 @@ static struct file *open_detached_copy(struct path *path, bool recursive)
 	return file;
 }
 
-SYSCALL_DEFINE3(open_tree, int, dfd, const char __user *, filename, unsigned, flags)
+static struct file *vfs_open_tree(int dfd, const char __user *filename, unsigned int flags)
 {
-	struct file *file;
-	struct path path;
+	int ret;
+	struct path path __free(path_put) = {};
 	int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
 	bool detached = flags & OPEN_TREE_CLONE;
-	int error;
-	int fd;
 
 	BUILD_BUG_ON(OPEN_TREE_CLOEXEC != O_CLOEXEC);
 
 	if (flags & ~(AT_EMPTY_PATH | AT_NO_AUTOMOUNT | AT_RECURSIVE |
 		      AT_SYMLINK_NOFOLLOW | OPEN_TREE_CLONE |
 		      OPEN_TREE_CLOEXEC))
-		return -EINVAL;
+		return ERR_PTR(-EINVAL);
 
 	if ((flags & (AT_RECURSIVE | OPEN_TREE_CLONE)) == AT_RECURSIVE)
-		return -EINVAL;
+		return ERR_PTR(-EINVAL);
 
 	if (flags & AT_NO_AUTOMOUNT)
 		lookup_flags &= ~LOOKUP_AUTOMOUNT;
@@ -3029,27 +3027,32 @@ SYSCALL_DEFINE3(open_tree, int, dfd, const char __user *, filename, unsigned, fl
 		lookup_flags |= LOOKUP_EMPTY;
 
 	if (detached && !may_mount())
-		return -EPERM;
+		return ERR_PTR(-EPERM);
+
+	ret = user_path_at(dfd, filename, lookup_flags, &path);
+	if (unlikely(ret))
+		return ERR_PTR(ret);
+
+	if (detached)
+		return open_detached_copy(&path, flags & AT_RECURSIVE);
+
+	return dentry_open(&path, O_PATH, current_cred());
+}
+
+SYSCALL_DEFINE3(open_tree, int, dfd, const char __user *, filename, unsigned, flags)
+{
+	int fd;
+	struct file *file __free(fput) = NULL;
+
+	file = vfs_open_tree(dfd, filename, flags);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
 
 	fd = get_unused_fd_flags(flags & O_CLOEXEC);
 	if (fd < 0)
 		return fd;
 
-	error = user_path_at(dfd, filename, lookup_flags, &path);
-	if (unlikely(error)) {
-		file = ERR_PTR(error);
-	} else {
-		if (detached)
-			file = open_detached_copy(&path, flags & AT_RECURSIVE);
-		else
-			file = dentry_open(&path, O_PATH, current_cred());
-		path_put(&path);
-	}
-	if (IS_ERR(file)) {
-		put_unused_fd(fd);
-		return PTR_ERR(file);
-	}
-	fd_install(fd, file);
+	fd_install(fd, no_free_ptr(file));
 	return fd;
 }
 

From 474f7825d5335798742b92f067e1d22365013107 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Tue, 28 Jan 2025 11:33:40 +0100
Subject: [PATCH 3/6] fs: add copy_mount_setattr() helper

Split out copy_mount_setattr() from mount_setattr() so we can use it in
later patches.

Link: https://lore.kernel.org/r/20250128-work-mnt_idmap-update-v2-v1-2-c25feb0d2eb3@kernel.org
Reviewed-by: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c | 73 +++++++++++++++++++++++++++-----------------------
 1 file changed, 40 insertions(+), 33 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 7604cdacba20b..d2ef1d69839be 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4814,7 +4814,7 @@ static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 }
 
 static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
-				struct mount_kattr *kattr, unsigned int flags)
+				struct mount_kattr *kattr)
 {
 	struct ns_common *ns;
 	struct user_namespace *mnt_userns;
@@ -4865,22 +4865,8 @@ static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
 }
 
 static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
-			     struct mount_kattr *kattr, unsigned int flags)
+			     struct mount_kattr *kattr)
 {
-	unsigned int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
-
-	if (flags & AT_NO_AUTOMOUNT)
-		lookup_flags &= ~LOOKUP_AUTOMOUNT;
-	if (flags & AT_SYMLINK_NOFOLLOW)
-		lookup_flags &= ~LOOKUP_FOLLOW;
-	if (flags & AT_EMPTY_PATH)
-		lookup_flags |= LOOKUP_EMPTY;
-
-	*kattr = (struct mount_kattr) {
-		.lookup_flags	= lookup_flags,
-		.recurse	= !!(flags & AT_RECURSIVE),
-	};
-
 	if (attr->propagation & ~MOUNT_SETATTR_PROPAGATION_FLAGS)
 		return -EINVAL;
 	if (hweight32(attr->propagation & MOUNT_SETATTR_PROPAGATION_FLAGS) > 1)
@@ -4928,7 +4914,7 @@ static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
 			return -EINVAL;
 	}
 
-	return build_mount_idmapped(attr, usize, kattr, flags);
+	return build_mount_idmapped(attr, usize, kattr);
 }
 
 static void finish_mount_kattr(struct mount_kattr *kattr)
@@ -4940,23 +4926,14 @@ static void finish_mount_kattr(struct mount_kattr *kattr)
 		mnt_idmap_put(kattr->mnt_idmap);
 }
 
-SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
-		unsigned int, flags, struct mount_attr __user *, uattr,
-		size_t, usize)
+static int copy_mount_setattr(struct mount_attr __user *uattr, size_t usize,
+			      struct mount_kattr *kattr)
 {
-	int err;
-	struct path target;
+	int ret;
 	struct mount_attr attr;
-	struct mount_kattr kattr;
 
 	BUILD_BUG_ON(sizeof(struct mount_attr) != MOUNT_ATTR_SIZE_VER0);
 
-	if (flags & ~(AT_EMPTY_PATH |
-		      AT_RECURSIVE |
-		      AT_SYMLINK_NOFOLLOW |
-		      AT_NO_AUTOMOUNT))
-		return -EINVAL;
-
 	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
 	if (unlikely(usize < MOUNT_ATTR_SIZE_VER0))
@@ -4965,9 +4942,9 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
 	if (!may_mount())
 		return -EPERM;
 
-	err = copy_struct_from_user(&attr, sizeof(attr), uattr, usize);
-	if (err)
-		return err;
+	ret = copy_struct_from_user(&attr, sizeof(attr), uattr, usize);
+	if (ret)
+		return ret;
 
 	/* Don't bother walking through the mounts if this is a nop. */
 	if (attr.attr_set == 0 &&
@@ -4975,7 +4952,37 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
 	    attr.propagation == 0)
 		return 0;
 
-	err = build_mount_kattr(&attr, usize, &kattr, flags);
+	return build_mount_kattr(&attr, usize, kattr);
+}
+
+SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
+		unsigned int, flags, struct mount_attr __user *, uattr,
+		size_t, usize)
+{
+	int err;
+	struct path target;
+	struct mount_kattr kattr;
+	unsigned int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
+
+	if (flags & ~(AT_EMPTY_PATH |
+		      AT_RECURSIVE |
+		      AT_SYMLINK_NOFOLLOW |
+		      AT_NO_AUTOMOUNT))
+		return -EINVAL;
+
+	if (flags & AT_NO_AUTOMOUNT)
+		lookup_flags &= ~LOOKUP_AUTOMOUNT;
+	if (flags & AT_SYMLINK_NOFOLLOW)
+		lookup_flags &= ~LOOKUP_FOLLOW;
+	if (flags & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
+
+	kattr = (struct mount_kattr) {
+		.lookup_flags	= lookup_flags,
+		.recurse	= !!(flags & AT_RECURSIVE),
+	};
+
+	err = copy_mount_setattr(uattr, usize, &kattr);
 	if (err)
 		return err;
 

From c4a16820d90199409c9bf01c4f794e1e9e8d8fd8 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Tue, 28 Jan 2025 11:33:41 +0100
Subject: [PATCH 4/6] 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      |  1 +
 arch/arm/tools/syscall.tbl                  |  1 +
 arch/arm64/tools/syscall_32.tbl             |  1 +
 arch/m68k/kernel/syscalls/syscall.tbl       |  1 +
 arch/microblaze/kernel/syscalls/syscall.tbl |  1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl   |  1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl   |  1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl   |  1 +
 arch/parisc/kernel/syscalls/syscall.tbl     |  1 +
 arch/powerpc/kernel/syscalls/syscall.tbl    |  1 +
 arch/s390/kernel/syscalls/syscall.tbl       |  1 +
 arch/sh/kernel/syscalls/syscall.tbl         |  1 +
 arch/sparc/kernel/syscalls/syscall.tbl      |  1 +
 arch/x86/entry/syscalls/syscall_32.tbl      |  1 +
 arch/x86/entry/syscalls/syscall_64.tbl      |  1 +
 arch/xtensa/kernel/syscalls/syscall.tbl     |  1 +
 fs/namespace.c                              | 39 +++++++++++++++++++++
 include/linux/syscalls.h                    |  4 +++
 include/uapi/asm-generic/unistd.h           |  4 ++-
 scripts/syscall.tbl                         |  1 +
 20 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index c59d53d6d3f34..2dd6340de6b4e 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index 49eeb2ad8dbd8..27c1d5ebcd91c 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/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
diff --git a/arch/arm64/tools/syscall_32.tbl b/arch/arm64/tools/syscall_32.tbl
index 69a829912a05e..0765b3a8d6d60 100644
--- a/arch/arm64/tools/syscall_32.tbl
+++ b/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
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index f5ed71f1910d0..9fe47112c586f 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 680f568b77f2c..7b6e97828e552 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index 0b9b7e25b69ad..aa70e371bb54a 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/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
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index c844cd5cda620..1e8c44c7b6149 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/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
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 349b8aad1159f..114a5a1a62302 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/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
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index d9fc94c869657..94df3cb957e9d 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index d8b4ab78bef07..9a084bdb89269 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index e9115b4d8b635..a4569b96ef06c 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index c8cad33bf250e..52a7652fcff63 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 727f99d333b30..83e45eb6c095a 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 4d0fb2fba7e20..3f0ec87d5db4e 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/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
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 5eb708bff1c79..cfb5ca41e30de 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/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
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 37effc1b134ee..f657a77314f86 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/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
diff --git a/fs/namespace.c b/fs/namespace.c
index d2ef1d69839be..ac4ad746c7705 100644
--- a/fs/namespace.c
+++ b/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)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index c6333204d4513..079ea1d09d85e 100644
--- a/include/linux/syscalls.h
+++ b/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);
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 88dc393c2bca3..2892a45023af6 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/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
diff --git a/scripts/syscall.tbl b/scripts/syscall.tbl
index ebbdb3c42e9f7..580b4e246aecd 100644
--- a/scripts/syscall.tbl
+++ b/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

From 325cca846fe4ed20fa68c076e25878ea9d350515 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Tue, 28 Jan 2025 11:33:42 +0100
Subject: [PATCH 5/6] fs: add kflags member to struct mount_kattr

Instead of using a boolean use a flag so we can add new flags in
following patches.

Link: https://lore.kernel.org/r/20250128-work-mnt_idmap-update-v2-v1-4-c25feb0d2eb3@kernel.org
Reviewed-by: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index ac4ad746c7705..a6d3f2041fda8 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -87,12 +87,16 @@ LIST_HEAD(notify_list); /* protected by namespace_sem */
 static struct rb_root mnt_ns_tree = RB_ROOT; /* protected by mnt_ns_tree_lock */
 static LIST_HEAD(mnt_ns_list); /* protected by mnt_ns_tree_lock */
 
+enum mount_kattr_flags_t {
+	MOUNT_KATTR_RECURSE		= (1 << 0),
+};
+
 struct mount_kattr {
 	unsigned int attr_set;
 	unsigned int attr_clr;
 	unsigned int propagation;
 	unsigned int lookup_flags;
-	bool recurse;
+	enum mount_kattr_flags_t kflags;
 	struct user_namespace *mnt_userns;
 	struct mnt_idmap *mnt_idmap;
 };
@@ -4672,7 +4676,7 @@ static int mount_setattr_prepare(struct mount_kattr *kattr, struct mount *mnt)
 				break;
 		}
 
-		if (!kattr->recurse)
+		if (!(kattr->kflags & MOUNT_KATTR_RECURSE))
 			return 0;
 	}
 
@@ -4733,7 +4737,7 @@ static void mount_setattr_commit(struct mount_kattr *kattr, struct mount *mnt)
 
 		if (kattr->propagation)
 			change_mnt_propagation(m, kattr->propagation);
-		if (!kattr->recurse)
+		if (!(kattr->kflags & MOUNT_KATTR_RECURSE))
 			break;
 	}
 	touch_mnt_namespace(mnt->mnt_ns);
@@ -4763,7 +4767,7 @@ static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 		 */
 		namespace_lock();
 		if (kattr->propagation == MS_SHARED) {
-			err = invent_group_ids(mnt, kattr->recurse);
+			err = invent_group_ids(mnt, kattr->kflags & MOUNT_KATTR_RECURSE);
 			if (err) {
 				namespace_unlock();
 				return err;
@@ -4979,9 +4983,11 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
 
 	kattr = (struct mount_kattr) {
 		.lookup_flags	= lookup_flags,
-		.recurse	= !!(flags & AT_RECURSIVE),
 	};
 
+	if (flags & AT_RECURSIVE)
+		kattr.kflags |= MOUNT_KATTR_RECURSE;
+
 	err = copy_mount_setattr(uattr, usize, &kattr);
 	if (err)
 		return err;
@@ -5011,9 +5017,10 @@ SYSCALL_DEFINE5(open_tree_attr, int, dfd, const char __user *, filename,
 
 	if (uattr) {
 		int ret;
-		struct mount_kattr kattr = {
-			.recurse = !!(flags & AT_RECURSIVE),
-		};
+		struct mount_kattr kattr = {};
+
+		if (flags & AT_RECURSIVE)
+			kattr.kflags |= MOUNT_KATTR_RECURSE;
 
 		ret = copy_mount_setattr(uattr, usize, &kattr);
 		if (ret)

From 2462651ffa76b87f9c2e4403ef6e6b89b703fb2f Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Tue, 28 Jan 2025 11:33:43 +0100
Subject: [PATCH 6/6] fs: allow changing idmappings

This patchset makes it possible to create a new idmapped mount from an
already idmapped mount and to clear idmappings.

// Create a first idmapped mount
struct mount_attr attr = {
        .attr_set = MOUNT_ATTR_IDMAP
        .userns_fd = fd_userns
};

fd_tree = open_tree(-EBADF, "/", OPEN_TREE_CLONE, &attr, sizeof(attr));
move_mount(fd_tree, "", -EBADF, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);

// Create a second idmapped mount from the first idmapped mount
attr.attr_set = MOUNT_ATTR_IDMAP;
attr.userns_fd = fd_userns2;
fd_tree2 = open_tree(-EBADF, "/mnt", OPEN_TREE_CLONE, &attr, sizeof(attr));

// Create a second non-idmapped mount from the first idmapped mount:
memset(&attr, 0, sizeof(attr));
attr.attr_clr = MOUNT_ATTR_IDMAP;
fd_tree2 = open_tree(-EBADF, "/mnt", OPEN_TREE_CLONE, &attr, sizeof(attr));

Link: https://lore.kernel.org/r/20250128-work-mnt_idmap-update-v2-v1-5-c25feb0d2eb3@kernel.org
Reviewed-by: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c | 53 ++++++++++++++++++++++++++++++--------------------
 1 file changed, 32 insertions(+), 21 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index a6d3f2041fda8..1d8d14ca6c13d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -89,6 +89,7 @@ static LIST_HEAD(mnt_ns_list); /* protected by mnt_ns_tree_lock */
 
 enum mount_kattr_flags_t {
 	MOUNT_KATTR_RECURSE		= (1 << 0),
+	MOUNT_KATTR_IDMAP_REPLACE	= (1 << 1),
 };
 
 struct mount_kattr {
@@ -4612,11 +4613,10 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 		return -EINVAL;
 
 	/*
-	 * Once a mount has been idmapped we don't allow it to change its
-	 * mapping. It makes things simpler and callers can just create
-	 * another bind-mount they can idmap if they want to.
+	 * We only allow an mount to change it's idmapping if it has
+	 * never been accessible to userspace.
 	 */
-	if (is_idmapped_mnt(m))
+	if (!(kattr->kflags & MOUNT_KATTR_IDMAP_REPLACE) && is_idmapped_mnt(m))
 		return -EPERM;
 
 	/* The underlying filesystem doesn't support idmapped mounts yet. */
@@ -4706,18 +4706,16 @@ static int mount_setattr_prepare(struct mount_kattr *kattr, struct mount *mnt)
 
 static void do_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 {
+	struct mnt_idmap *old_idmap;
+
 	if (!kattr->mnt_idmap)
 		return;
 
-	/*
-	 * Pairs with smp_load_acquire() in mnt_idmap().
-	 *
-	 * Since we only allow a mount to change the idmapping once and
-	 * verified this in can_idmap_mount() we know that the mount has
-	 * @nop_mnt_idmap attached to it. So there's no need to drop any
-	 * references.
-	 */
+	old_idmap = mnt_idmap(&mnt->mnt);
+
+	/* Pairs with smp_load_acquire() in mnt_idmap(). */
 	smp_store_release(&mnt->mnt.mnt_idmap, mnt_idmap_get(kattr->mnt_idmap));
+	mnt_idmap_put(old_idmap);
 }
 
 static void mount_setattr_commit(struct mount_kattr *kattr, struct mount *mnt)
@@ -4826,13 +4824,23 @@ static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
 	if (!((attr->attr_set | attr->attr_clr) & MOUNT_ATTR_IDMAP))
 		return 0;
 
-	/*
-	 * We currently do not support clearing an idmapped mount. If this ever
-	 * is a use-case we can revisit this but for now let's keep it simple
-	 * and not allow it.
-	 */
-	if (attr->attr_clr & MOUNT_ATTR_IDMAP)
-		return -EINVAL;
+	if (attr->attr_clr & MOUNT_ATTR_IDMAP) {
+		/*
+		 * We can only remove an idmapping if it's never been
+		 * exposed to userspace.
+		 */
+		if (!(kattr->kflags & MOUNT_KATTR_IDMAP_REPLACE))
+			return -EINVAL;
+
+		/*
+		 * Removal of idmappings is equivalent to setting
+		 * nop_mnt_idmap.
+		 */
+		if (!(attr->attr_set & MOUNT_ATTR_IDMAP)) {
+			kattr->mnt_idmap = &nop_mnt_idmap;
+			return 0;
+		}
+	}
 
 	if (attr->userns_fd > INT_MAX)
 		return -EINVAL;
@@ -4923,8 +4931,10 @@ static int build_mount_kattr(const struct mount_attr *attr, size_t usize,
 
 static void finish_mount_kattr(struct mount_kattr *kattr)
 {
-	put_user_ns(kattr->mnt_userns);
-	kattr->mnt_userns = NULL;
+	if (kattr->mnt_userns) {
+		put_user_ns(kattr->mnt_userns);
+		kattr->mnt_userns = NULL;
+	}
 
 	if (kattr->mnt_idmap)
 		mnt_idmap_put(kattr->mnt_idmap);
@@ -5019,6 +5029,7 @@ SYSCALL_DEFINE5(open_tree_attr, int, dfd, const char __user *, filename,
 		int ret;
 		struct mount_kattr kattr = {};
 
+		kattr.kflags = MOUNT_KATTR_IDMAP_REPLACE;
 		if (flags & AT_RECURSIVE)
 			kattr.kflags |= MOUNT_KATTR_RECURSE;