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Christian Brauner
fa0d6d945e
nullfs provides a permanently empty and immutable directory. Lookups
fail with ENOENT. The directory can be opened, read, stat, mounted upon.
It behaves like nothing is there.
Add its counterpart failfs where the semantics are not "there is
nothing here" but "nothing is supported here". Every operation that
reaches the filesystem fails with EOPNOTSUPP. Even statfs()/fstatfs()
fail so the filesystem cannot be discovered through an fd to it.
EOPNOTSUPP rather than a permission errno keeps that coherent. There
is no permission model in which anything could ever be allowed and
EACCES or EPERM would merely suggest that different credentials might
succeed while EIO would suggest corruption. It also makes hitting the
failfs boundary mostly quite dinstinguishable. A task anchoring its
lookups at real directory file descriptors may be able to tell a failfs
refusal from an ordinary permission failure. I wouldn't go so far as
guaranteeing that but it should mostly work.
No path lookup can open the root, not even with O_PATH. It is never
reached by a lookup in a parent directory. The only way to a path-walk
terminal at the root is a jump through a /proc/<pid>/{root,cwd} magic
link or by mountpoint traversal. The root also refuses
->d_weak_revalidate() which the VFS calls for jumped terminals. That
covers the jump-based references too: an O_PATH open is refused,
name_to_handle_at() cannot encode it into a file handle, and following a
magic link into it fails. A plain readlink() of such a link still works
and shows "failfs:/".
There is a single instance of failfs mounted during early boot via
kern_mount() making it logically distinct from every mount namespace.
Since the mount is a member of no mount namespace mounting onto it
fails. So nothing can ever be mounted on top of it. It cannot be cloned
via OPEN_TREE_CLONE and it does not show up in statmount()/listmount()
or /proc/<pid>/mountinfo. The filesystem is not registered so it is
not visible in /proc/filesystems and cannot be mounted from userspace.
This lets tasks shed their filesystem state completely. A process with
its root directory or working directory in failfs must anchor every path
lookup at an explicit file descriptor or is doomed to fail any lookup.
Absolute paths, absolute symlinks, and AT_FDCWD-relative lookups
simply fail. Followup patches will expose it via a new FD_FAILFS_ROOT
file descriptor sentinel understood by fchdir() and the new fchroot()
system call.
Link: https://patch.msgid.link/20260724-work-failfs-v2-1-485dabbae185@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
111 lines
4.0 KiB
C
111 lines
4.0 KiB
C
/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
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#ifndef __LINUX_MAGIC_H__
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#define __LINUX_MAGIC_H__
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#define ADFS_SUPER_MAGIC 0xadf5
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#define AFFS_SUPER_MAGIC 0xadff
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#define AFS_SUPER_MAGIC 0x5346414F
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#define AUTOFS_SUPER_MAGIC 0x0187
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#define CEPH_SUPER_MAGIC 0x00c36400
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#define CODA_SUPER_MAGIC 0x73757245
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#define CRAMFS_MAGIC 0x28cd3d45 /* some random number */
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#define CRAMFS_MAGIC_WEND 0x453dcd28 /* magic number with the wrong endianess */
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#define DEBUGFS_MAGIC 0x64626720
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#define SECURITYFS_MAGIC 0x73636673
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#define SELINUX_MAGIC 0xf97cff8c
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#define SMACK_MAGIC 0x43415d53 /* "SMAC" */
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#define RAMFS_MAGIC 0x858458f6 /* some random number */
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#define TMPFS_MAGIC 0x01021994
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#define HUGETLBFS_MAGIC 0x958458f6 /* some random number */
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#define SQUASHFS_MAGIC 0x73717368
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#define ECRYPTFS_SUPER_MAGIC 0xf15f
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#define EFS_SUPER_MAGIC 0x414A53
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#define EROFS_SUPER_MAGIC_V1 0xE0F5E1E2
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#define EXT2_SUPER_MAGIC 0xEF53
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#define EXT3_SUPER_MAGIC 0xEF53
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#define XENFS_SUPER_MAGIC 0xabba1974
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#define EXT4_SUPER_MAGIC 0xEF53
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#define BTRFS_SUPER_MAGIC 0x9123683E
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#define NILFS_SUPER_MAGIC 0x3434
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#define F2FS_SUPER_MAGIC 0xF2F52010
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#define HPFS_SUPER_MAGIC 0xf995e849
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#define ISOFS_SUPER_MAGIC 0x9660
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#define JFFS2_SUPER_MAGIC 0x72b6
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#define XFS_SUPER_MAGIC 0x58465342 /* "XFSB" */
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#define PSTOREFS_MAGIC 0x6165676C
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#define EFIVARFS_MAGIC 0xde5e81e4
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#define HOSTFS_SUPER_MAGIC 0x00c0ffee
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#define OVERLAYFS_SUPER_MAGIC 0x794c7630
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#define FUSE_SUPER_MAGIC 0x65735546
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#define BCACHEFS_SUPER_MAGIC 0xca451a4e
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#define MINIX_SUPER_MAGIC 0x137F /* minix v1 fs, 14 char names */
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#define MINIX_SUPER_MAGIC2 0x138F /* minix v1 fs, 30 char names */
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#define MINIX2_SUPER_MAGIC 0x2468 /* minix v2 fs, 14 char names */
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#define MINIX2_SUPER_MAGIC2 0x2478 /* minix v2 fs, 30 char names */
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#define MINIX3_SUPER_MAGIC 0x4d5a /* minix v3 fs, 60 char names */
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#define MSDOS_SUPER_MAGIC 0x4d44 /* MD */
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#define EXFAT_SUPER_MAGIC 0x2011BAB0
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#define NCP_SUPER_MAGIC 0x564c /* Guess, what 0x564c is :-) */
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#define NFS_SUPER_MAGIC 0x6969
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#define OCFS2_SUPER_MAGIC 0x7461636f
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#define OPENPROM_SUPER_MAGIC 0x9fa1
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#define QNX4_SUPER_MAGIC 0x002f /* qnx4 fs detection */
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#define QNX6_SUPER_MAGIC 0x68191122 /* qnx6 fs detection */
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#define AFS_FS_MAGIC 0x6B414653
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#define REISERFS_SUPER_MAGIC 0x52654973 /* used by gcc */
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/* used by file system utilities that
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look at the superblock, etc. */
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#define REISERFS_SUPER_MAGIC_STRING "ReIsErFs"
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#define REISER2FS_SUPER_MAGIC_STRING "ReIsEr2Fs"
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#define REISER2FS_JR_SUPER_MAGIC_STRING "ReIsEr3Fs"
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#define SMB_SUPER_MAGIC 0x517B
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#define CIFS_SUPER_MAGIC 0xFF534D42 /* the first four bytes of SMB PDUs */
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#define SMB2_SUPER_MAGIC 0xFE534D42
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#define CGROUP_SUPER_MAGIC 0x27e0eb
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#define CGROUP2_SUPER_MAGIC 0x63677270
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#define RDTGROUP_SUPER_MAGIC 0x7655821
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#define STACK_END_MAGIC 0x57AC6E9D
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#define TRACEFS_MAGIC 0x74726163
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#define V9FS_MAGIC 0x01021997
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#define BDEVFS_MAGIC 0x62646576
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#define DAXFS_MAGIC 0x64646178
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#define BINFMTFS_MAGIC 0x42494e4d
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#define DEVPTS_SUPER_MAGIC 0x1cd1
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#define BINDERFS_SUPER_MAGIC 0x6c6f6f70
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#define FUTEXFS_SUPER_MAGIC 0xBAD1DEA
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#define PIPEFS_MAGIC 0x50495045
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#define PROC_SUPER_MAGIC 0x9fa0
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#define SOCKFS_MAGIC 0x534F434B
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#define SYSFS_MAGIC 0x62656572
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#define USBDEVICE_SUPER_MAGIC 0x9fa2
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#define MTD_INODE_FS_MAGIC 0x11307854
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#define ANON_INODE_FS_MAGIC 0x09041934
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#define BTRFS_TEST_MAGIC 0x73727279
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#define NSFS_MAGIC 0x6e736673
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#define BPF_FS_MAGIC 0xcafe4a11
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#define AAFS_MAGIC 0x5a3c69f0
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#define ZONEFS_MAGIC 0x5a4f4653
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/* Since UDF 2.01 is ISO 13346 based... */
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#define UDF_SUPER_MAGIC 0x15013346
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#define DMA_BUF_MAGIC 0x444d4142 /* "DMAB" */
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#define DEVMEM_MAGIC 0x454d444d /* "DMEM" */
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#define SECRETMEM_MAGIC 0x5345434d /* "SECM" */
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#define PID_FS_MAGIC 0x50494446 /* "PIDF" */
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#define GUEST_MEMFD_MAGIC 0x474d454d /* "GMEM" */
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#define NULL_FS_MAGIC 0x4E554C4C /* "NULL" */
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#define FAIL_FS_MAGIC 0x4641494C /* "FAIL" */
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#endif /* __LINUX_MAGIC_H__ */
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