2a1867219c
This implements the missing mount_setattr() syscall. While the new mount
api allows to change the properties of a superblock there is currently
no way to change the properties of a mount or a mount tree using file
descriptors which the new mount api is based on. In addition the old
mount api has the restriction that mount options cannot be applied
recursively. This hasn't changed since changing mount options on a
per-mount basis was implemented in [1] and has been a frequent request
not just for convenience but also for security reasons. The legacy
mount syscall is unable to accommodate this behavior without introducing
a whole new set of flags because MS_REC | MS_REMOUNT | MS_BIND |
MS_RDONLY | MS_NOEXEC | [...] only apply the mount option to the topmost
mount. Changing MS_REC to apply to the whole mount tree would mean
introducing a significant uapi change and would likely cause significant
regressions.
The new mount_setattr() syscall allows to recursively clear and set
mount options in one shot. Multiple calls to change mount options
requesting the same changes are idempotent:
int mount_setattr(int dfd, const char *path, unsigned flags,
struct mount_attr *uattr, size_t usize);
Flags to modify path resolution behavior are specified in the @flags
argument. Currently, AT_EMPTY_PATH, AT_RECURSIVE, AT_SYMLINK_NOFOLLOW,
and AT_NO_AUTOMOUNT are supported. If useful, additional lookup flags to
restrict path resolution as introduced with openat2() might be supported
in the future.
The mount_setattr() syscall can be expected to grow over time and is
designed with extensibility in mind. It follows the extensible syscall
pattern we have used with other syscalls such as openat2(), clone3(),
sched_{set,get}attr(), and others.
The set of mount options is passed in the uapi struct mount_attr which
currently has the following layout:
struct mount_attr {
__u64 attr_set;
__u64 attr_clr;
__u64 propagation;
__u64 userns_fd;
};
The @attr_set and @attr_clr members are used to clear and set mount
options. This way a user can e.g. request that a set of flags is to be
raised such as turning mounts readonly by raising MOUNT_ATTR_RDONLY in
@attr_set while at the same time requesting that another set of flags is
to be lowered such as removing noexec from a mount tree by specifying
MOUNT_ATTR_NOEXEC in @attr_clr.
Note, since the MOUNT_ATTR_<atime> values are an enum starting from 0,
not a bitmap, users wanting to transition to a different atime setting
cannot simply specify the atime setting in @attr_set, but must also
specify MOUNT_ATTR__ATIME in the @attr_clr field. So we ensure that
MOUNT_ATTR__ATIME can't be partially set in @attr_clr and that @attr_set
can't have any atime bits set if MOUNT_ATTR__ATIME isn't set in
@attr_clr.
The @propagation field lets callers specify the propagation type of a
mount tree. Propagation is a single property that has four different
settings and as such is not really a flag argument but an enum.
Specifically, it would be unclear what setting and clearing propagation
settings in combination would amount to. The legacy mount() syscall thus
forbids the combination of multiple propagation settings too. The goal
is to keep the semantics of mount propagation somewhat simple as they
are overly complex as it is.
The @userns_fd field lets user specify a user namespace whose idmapping
becomes the idmapping of the mount. This is implemented and explained in
detail in the next patch.
[1]: commit 2e4b7fcd92 ("[PATCH] r/o bind mounts: honor mount writer counts at remount")
Link: https://lore.kernel.org/r/20210121131959.646623-35-christian.brauner@ubuntu.com
Cc: David Howells <dhowells@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
51 lines
1.3 KiB
C
51 lines
1.3 KiB
C
/* SPDX-License-Identifier: GPL-2.0-only */
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/*
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* Copyright (C) 2012 ARM Ltd.
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*/
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#ifdef CONFIG_COMPAT
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#define __ARCH_WANT_COMPAT_STAT64
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#define __ARCH_WANT_SYS_GETHOSTNAME
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#define __ARCH_WANT_SYS_PAUSE
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#define __ARCH_WANT_SYS_GETPGRP
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#define __ARCH_WANT_SYS_NICE
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#define __ARCH_WANT_SYS_SIGPENDING
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#define __ARCH_WANT_SYS_SIGPROCMASK
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#define __ARCH_WANT_COMPAT_SYS_SENDFILE
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#define __ARCH_WANT_SYS_UTIME32
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#define __ARCH_WANT_SYS_FORK
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#define __ARCH_WANT_SYS_VFORK
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/*
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* Compat syscall numbers used by the AArch64 kernel.
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*/
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#define __NR_compat_restart_syscall 0
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#define __NR_compat_exit 1
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#define __NR_compat_read 3
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#define __NR_compat_write 4
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#define __NR_compat_gettimeofday 78
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#define __NR_compat_sigreturn 119
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#define __NR_compat_rt_sigreturn 173
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#define __NR_compat_clock_gettime 263
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#define __NR_compat_clock_getres 264
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#define __NR_compat_clock_gettime64 403
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#define __NR_compat_clock_getres_time64 406
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/*
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* The following SVCs are ARM private.
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*/
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#define __ARM_NR_COMPAT_BASE 0x0f0000
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#define __ARM_NR_compat_cacheflush (__ARM_NR_COMPAT_BASE + 2)
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#define __ARM_NR_compat_set_tls (__ARM_NR_COMPAT_BASE + 5)
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#define __ARM_NR_COMPAT_END (__ARM_NR_COMPAT_BASE + 0x800)
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#define __NR_compat_syscalls 443
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#endif
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#define __ARCH_WANT_SYS_CLONE
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#ifndef __COMPAT_SYSCALL_NR
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#include <uapi/asm/unistd.h>
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#endif
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#define NR_syscalls (__NR_syscalls)
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