[PATCH 00/25] VFS: Introduce filesystem information query syscall [ver #14]

From: David Howells
Date: Mon Jun 24 2019 - 10:08:56 EST

Hi Al,

Here are a set of patches that adds a syscall, fsinfo(), that allows
attributes of a filesystem/superblock to be queried. Attribute values are
of four basic types:

(1) Version dependent-length structure (size defined by type).

(2) Variable-length string (up to PAGE_SIZE).

(3) Array of fixed-length structures (up to INT_MAX size).

(4) Opaque blob (up to INT_MAX size).

Attributes can have multiple values in up to two dimensions and all the
values of a particular attribute must have the same type.

Note that the attribute values *are* allowed to vary between dentries
within a single superblock, depending on the specific dentry that you're
looking at.

I've tried to make the interface as light as possible, so integer/enum
attribute selector rather than string and the core does all the allocation
and extensibility support work rather than leaving that to the filesystems.
That means that for the first two attribute types, sb->s_op->fsinfo() may
assume that the provided buffer is always present and always big enough.

Further, this removes the possibility of the filesystem gaining access to the
userspace buffer.

fsinfo() allows a variety of information to be retrieved about a filesystem
and the mount topology:

(1) General superblock attributes:

- The amount of space/free space in a filesystem (as statfs()).
- Filesystem identifiers (UUID, volume label, device numbers, ...)
- The limits on a filesystem's capabilities
- Information on supported statx fields and attributes and IOC flags.
- A variety single-bit flags indicating supported capabilities.
- Timestamp resolution and range.
- Sources (as per mount(2), but fsconfig() allows multiple sources).
- In-filesystem filename format information.
- Filesystem parameters ("mount -o xxx"-type things).
- LSM parameters (again "mount -o xxx"-type things).

(2) Filesystem-specific superblock attributes:

- Server names and addresses.
- Cell name.

(3) Filesystem configuration metadata attributes:

- Filesystem parameter type descriptions.
- Name -> parameter mappings.
- Simple enumeration name -> value mappings.

(4) Mount topology:

- General information about a mount object.
- Mount device name(s).
- Children of a mount object and their relative paths.

(5) Information about what the fsinfo() syscall itself supports, including
the number of attibutes supported and the number of capability bits

The system is extensible:

(1) New attributes can be added. There is no requirement that a
filesystem implement every attribute. Note that the core VFS keeps a
table of types and sizes so it can handle future extensibility rather
than delegating this to the filesystems.

(2) Version length-dependent structure attributes can be made larger and
have additional information tacked on the end, provided it keeps the
layout of the existing fields. If an older process asks for a shorter
structure, it will only be given the bits it asks for. If a newer
process asks for a longer structure on an older kernel, the extra
space will be set to 0. In all cases, the size of the data actually
available is returned.

In essence, the size of a structure is that structure's version: a
smaller size is an earlier version and a later version includes
everything that the earlier version did.

(3) New single-bit capability flags can be added. This is a structure-typed
attribute and, as such, (2) applies. Any bits you wanted but the kernel
doesn't support are automatically set to 0.

If a filesystem-specific attribute is added, it should just take up the next
number in the enumeration. Currently, I do not intend that the number space
should be subdivided between interested parties.

fsinfo() may be called like the following, for example:

struct fsinfo_params params = {
.Nth = 2;
.Mth = 1;
struct fsinfo_server_address address;

len = fsinfo(AT_FDCWD, "/afs/grand.central.org/doc", &params,
&address, sizeof(address));

The above example would query a network filesystem, such as AFS or NFS, and
ask what the 2nd address (Mth) of the 3rd server (Nth) that the superblock is
using is. Whereas:

struct fsinfo_params params = {
char cell_name[256];

len = fsinfo(AT_FDCWD, "/afs/grand.central.org/doc", &params,
&cell_name, sizeof(cell_name));

would retrieve the name of an AFS cell as a string.

fsinfo() can also be used to query a context from fsopen() or fspick():

fd = fsopen("ext4", 0);
struct fsinfo_params params = {
struct fsinfo_param_description desc;
fsinfo(fd, NULL, &params, &desc, sizeof(desc));

even if that context doesn't currently have a superblock attached (though if
there's no superblock attached, only filesystem-specific things like parameter
descriptions can be accessed).

The patches can be found here also:


on branch:



ver #14:

(*) Increase to 128-bit the fields for number of blocks and files in the
filesystem and also the max file size and max inode number fields.

(*) Increase to 64-bit the fields for max hard links and max xattr body

(*) Provide struct fsinfo_timestamp_one to represent the characteristics
of a single timestamp and move the range into it. FAT, for example,
has different ranges for different timestamps. Each timestamp is then
represented by one of these structs.

(*) Don't expose MS_* flags (such as MS_RDONLY) through this interface as
they ought to be considered deprecated; instead anyone who wants them
should parse FSINFO_ATTR_PARAMETERS for the string equivalents.

(*) Add a flag, AT_FSINFO_FROM_FSOPEN, to indicate that the fd being
accessed is from fsopen()/fspick() and that fsinfo() should look
inside and access the filesystem referred to by the fs_context.

(*) If the filesystem implements FSINFO_ATTR_PARAMETERS for itself, don't
automatically include flags for the SB_* bits that are normally
rendered by, say, /proc/mounts (such as SB_RDONLY). Rather, a helper
is provided that the filesystem must call with an appropriately
wangled s_flags.

(*) Drop the NFS fsinfo patch for now as NFS fs_context support is
unlikely to get upstream in the upcoming merge window.

ver #13:

(*) Provided a "fixed-struct array" type so that the list of children of a
mount and all their change counters can be read atomically.

(*) Additional filesystem examples.

(*) Documented the API.

ver #12:

(*) Rename ->get_fsinfo() to ->fsinfo().

(*) Pass the path through to to ->fsinfo() as it's needed for NFS to
retrocalculate the source name.

(*) Indicated which is the source parameter in the param-description

(*) Dropped the realm attribute.

David Howells (17):
vfs: syscall: Add fsinfo() to query filesystem information
fsinfo: Add syscalls to other arches
vfs: Allow fsinfo() to query what's in an fs_context
vfs: Allow fsinfo() to be used to query an fs parameter description
vfs: Implement parameter value retrieval with fsinfo()
fsinfo: Implement retrieval of LSM parameters with fsinfo()
vfs: Introduce a non-repeating system-unique superblock ID
vfs: Allow fsinfo() to look up a mount object by ID
vfs: Add mount notification count
vfs: Allow mount information to be queried by fsinfo()
vfs: fsinfo sample: Mount listing program
fsinfo: Add API documentation
hugetlbfs: Add support for fsinfo()
kernfs, cgroup: Add fsinfo support
fsinfo: Support SELinux superblock parameter retrieval
fsinfo: Support Smack superblock parameter retrieval
afs: Support fsinfo()

Ian Kent (8):
fsinfo: proc - add sb operation fsinfo()
fsinfo: autofs - add sb operation fsinfo()
fsinfo: shmem - add tmpfs sb operation fsinfo()
fsinfo: devpts - add sb operation fsinfo()
fsinfo: pstore - add sb operation fsinfo()
fsinfo: debugfs - add sb operation fsinfo()
fsinfo: bpf - add sb operation fsinfo()
fsinfo: ufs - add sb operation fsinfo()

Documentation/filesystems/fsinfo.rst | 596 ++++++++++++++++++
arch/alpha/kernel/syscalls/syscall.tbl | 1
arch/arm/tools/syscall.tbl | 1
arch/arm64/include/asm/unistd.h | 2
arch/ia64/kernel/syscalls/syscall.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/Kconfig | 7
fs/Makefile | 1
fs/afs/internal.h | 1
fs/afs/super.c | 180 +++++-
fs/autofs/inode.c | 64 ++
fs/d_path.c | 2
fs/debugfs/inode.c | 38 +
fs/devpts/inode.c | 43 +
fs/fsinfo.c | 877 +++++++++++++++++++++++++++
fs/hugetlbfs/inode.c | 57 ++
fs/internal.h | 11
fs/kernfs/mount.c | 20 +
fs/mount.h | 22 +
fs/namespace.c | 307 +++++++++
fs/proc/inode.c | 37 +
fs/pstore/inode.c | 32 +
fs/statfs.c | 2
fs/super.c | 24 +
fs/ufs/super.c | 58 ++
include/linux/fs.h | 8
include/linux/fsinfo.h | 70 ++
include/linux/kernfs.h | 4
include/linux/lsm_hooks.h | 13
include/linux/security.h | 11
include/linux/syscalls.h | 4
include/uapi/asm-generic/unistd.h | 4
include/uapi/linux/fcntl.h | 3
include/uapi/linux/fsinfo.h | 319 ++++++++++
kernel/bpf/inode.c | 25 +
kernel/cgroup/cgroup-v1.c | 44 +
kernel/cgroup/cgroup.c | 19 +
kernel/sys_ni.c | 1
mm/shmem.c | 72 ++
samples/vfs/Makefile | 9
samples/vfs/test-fs-query.c | 138 ++++
samples/vfs/test-fsinfo.c | 682 +++++++++++++++++++++
samples/vfs/test-mntinfo.c | 241 +++++++
security/security.c | 12
security/selinux/hooks.c | 41 +
security/selinux/include/security.h | 2
security/selinux/ss/services.c | 49 ++
security/smack/smack_lsm.c | 43 +
60 files changed, 4202 insertions(+), 9 deletions(-)
create mode 100644 Documentation/filesystems/fsinfo.rst
create mode 100644 fs/fsinfo.c
create mode 100644 include/linux/fsinfo.h
create mode 100644 include/uapi/linux/fsinfo.h
create mode 100644 samples/vfs/test-fs-query.c
create mode 100644 samples/vfs/test-fsinfo.c
create mode 100644 samples/vfs/test-mntinfo.c