Message-Id: <20250919-new-mount-api-v4-9-1261201ab562@cyphar.com>
Date: Fri, 19 Sep 2025 11:59:50 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Alejandro Colomar <alx@...nel.org>
Cc: "Michael T. Kerrisk" <mtk.manpages@...il.com>, 
 Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
 Askar Safin <safinaskar@...omail.com>, 
 "G. Branden Robinson" <g.branden.robinson@...il.com>, 
 linux-man@...r.kernel.org, linux-api@...r.kernel.org, 
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
 David Howells <dhowells@...hat.com>, Christian Brauner <brauner@...nel.org>, 
 Aleksa Sarai <cyphar@...har.com>
Subject: [PATCH v4 09/10] man/man2/open_tree{,_attr}.2: document new
 open_tree_attr() API

This is a new API added in Linux 6.15, and is effectively just a minor
expansion of open_tree(2) in order to allow for MOUNT_ATTR_IDMAP to be
changed for an existing ID-mapped mount.  glibc does not yet have a
wrapper for this.

While working on this man-page, I discovered a bug in open_tree_attr(2)
that accidentally permitted changing MOUNT_ATTR_IDMAP for extant
detached ID-mapped mount objects.  This is definitely a bug, but there
is no need to add this to BUGS because the patch to fix this has already
been accepted (slated for 6.18, and will be backported to 6.15+).

Cc: Christian Brauner <brauner@...nel.org>
Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 man/man2/open_tree.2      | 140 ++++++++++++++++++++++++++++++++++++++++++++++
 man/man2/open_tree_attr.2 |   1 +
 2 files changed, 141 insertions(+)

diff --git a/man/man2/open_tree.2 b/man/man2/open_tree.2
index 7f85df08b43c7b48a9d021dbbeb2c60092a2b2d4..60de4313a9d5be4ef3ff1217051f252506a2ade9 100644
--- a/man/man2/open_tree.2
+++ b/man/man2/open_tree.2
@@ -15,7 +15,19 @@ .SH SYNOPSIS
 .B #include <sys/mount.h>
 .P
 .BI "int open_tree(int " dirfd ", const char *" path ", unsigned int " flags );
+.P
+.BR "#include <sys/syscall.h>" "    /* Definition of " SYS_* " constants */"
+.P
+.BI "int syscall(SYS_open_tree_attr, int " dirfd ", const char *" path ,
+.BI "            unsigned int " flags ", struct mount_attr *_Nullable " attr ", \
+size_t " size );
 .fi
+.P
+.IR Note :
+glibc provides no wrapper for
+.BR open_tree_attr (),
+necessitating the use of
+.BR syscall (2).
 .SH DESCRIPTION
 The
 .BR open_tree ()
@@ -246,6 +258,129 @@ .SH DESCRIPTION
 as a detached mount object.
 This flag is only permitted in conjunction with
 .BR \%OPEN_TREE_CLONE .
+.SS open_tree_attr()
+The
+.BR open_tree_attr ()
+system call operates in exactly the same way as
+.BR open_tree (),
+except for the differences described here.
+.P
+After performing the same operation as with
+.BR open_tree (),
+.BR open_tree_attr ()
+will apply the mount attribute changes described in
+.I attr
+to the file descriptor before it is returned.
+(See
+.BR mount_attr (2type)
+for a description of the
+.I mount_attr
+structure.
+As described in
+.BR mount_setattr (2),
+.I size
+must be set to
+.I sizeof(struct mount_attr)
+in order to support future extensions.)
+If
+.I attr
+is NULL,
+or has
+.IR attr.attr_clr ,
+.IR attr.attr_set ,
+and
+.I attr.propagation
+all set to zero,
+then
+.BR open_tree_attr ()
+has identical behaviour to
+.BR open_tree ().
+.P
+The application of
+.I attr
+to the resultant file descriptor
+has identical semantics to
+.BR mount_setattr (2),
+except for the following extensions and general caveats:
+.IP \[bu] 3
+Unlike
+.BR mount_setattr (2)
+called with a regular
+.B OPEN_TREE_CLONE
+detached mount object from
+.BR open_tree (),
+.BR open_tree_attr ()
+can specify a different setting for
+.B \%MOUNT_ATTR_IDMAP
+to the original mount object cloned with
+.BR OPEN_TREE_CLONE .
+.IP
+Adding
+.B \%MOUNT_ATTR_IDMAP
+to
+.I attr.attr_clr
+will disable ID-mapping for the new mount object;
+adding
+.B \%MOUNT_ATTR_IDMAP
+to
+.I attr.attr_set
+will configure the mount object to have the ID-mapping defined by
+the user namespace referenced by the file descriptor
+.IR attr.userns_fd .
+(The semantics of which are identical to when
+.BR mount_setattr (2)
+is used to configure
+.BR \%MOUNT_ATTR_IDMAP .)
+.IP
+Changing or removing the mapping
+of an ID-mapped mount is only permitted
+if a new detached mount object is being created with
+.I flags
+including
+.BR \%OPEN_TREE_CLONE .
+.\" Aleksa Sarai
+.\"  At time of writing, this is not actually true because of a bug where
+.\"  open_tree_attr() would accidentally permit changing MOUNT_ATTR_IDMAP for
+.\"  existing detached mount objects without setting OPEN_TREE_CLONE, but a
+.\"  patch to fix it has been slated for 6.18 and will be backported to 6.15+.
+.\"  <https://lore.kernel.org/r/20250808-open_tree_attr-bugfix-idmap-v1-0-0ec7bc05646c@cyphar.com/>
+.IP \[bu]
+If
+.I flags
+contains
+.BR \%AT_RECURSIVE ,
+then the attributes described in
+.I attr
+are applied recursively
+(just as when
+.BR mount_setattr (2)
+is called with
+.BR \%AT_RECURSIVE ).
+However, this applies in addition to the
+.BR open_tree ()-specific
+behaviour regarding
+.BR \%AT_RECURSIVE ,
+and thus
+.I flags
+must also contain
+.BR \%OPEN_TREE_CLONE .
+.P
+Note that if
+.I flags
+does not contain
+.BR \%OPEN_TREE_CLONE ,
+.BR open_tree_attr ()
+will attempt to modify the mount attributes of
+the mount object attached at
+the path described by
+.I dirfd
+and
+.IR path .
+As with
+.BR mount_setattr (2),
+if said path is not a mount point,
+.BR open_tree_attr ()
+will return an error.
 .SH RETURN VALUE
 On success, a new file descriptor is returned.
 On error, \-1 is returned, and
@@ -339,10 +474,15 @@ .SH ERRORS
 .SH STANDARDS
 Linux.
 .SH HISTORY
+.SS open_tree()
 Linux 5.2.
 .\" commit a07b20004793d8926f78d63eb5980559f7813404
 .\" commit 400913252d09f9cfb8cce33daee43167921fc343
 glibc 2.36.
+.SS open_tree_attr()
+Linux 6.15.
+.\" commit c4a16820d90199409c9bf01c4f794e1e9e8d8fd8
+.\" commit 7a54947e727b6df840780a66c970395ed9734ebe
 .SH NOTES
 .SS Mount propagation
 The bind-mount mount objects created by
diff --git a/man/man2/open_tree_attr.2 b/man/man2/open_tree_attr.2
new file mode 100644
index 0000000000000000000000000000000000000000..e57269bbd269bcce0b0a974425644ba75e379f2f
--- /dev/null
+++ b/man/man2/open_tree_attr.2
@@ -0,0 +1 @@
+.so man2/open_tree.2

-- 
2.51.0

Powered by blists - more mailing lists