vfs: Add permission flags for setting file attributes

Some permission models can allow processes to take ownership of a file,
change the file permissions, and set the file timestamps.  Introduce new
permission mask flags and check for those permissions in
inode_change_ok().

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
vfs: Make acl_permission_check() work for richacls

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: In-memory representation and helper functions

A richacl consists of an NFSv4 acl and an owner, group, and other mask.
These three masks correspond to the owner, group, and other file
permission bits, but they contain NFSv4 permissions instead of POSIX
permissions.

Each entry in the NFSv4 acl applies to the file owner (OWNER@), the
owning group (GROUP@), literally everyone (EVERYONE@), or to a specific
uid or gid.

As in the standard POSIX file permission model, each process is the
owner, group, or other file class.  A richacl grants a requested access
only if the NFSv4 acl in the richacl grants the access (according to the
NFSv4 permission check algorithm), and the file mask that applies to the
process includes the requested permissions.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: Permission mapping functions

We need to map from POSIX permissions to NFSv4 permissions when a
chmod() is done, from NFSv4 permissions to POSIX permissions when an acl
is set (which implicitly sets the file permission bits), and from the
MAY_READ/MAY_WRITE/MAY_EXEC/MAY_APPEND flags to NFSv4 permissions when
doing an access check in a richacl.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: Compute maximum file masks from an acl

Compute upper bound owner, group, and other file masks with as few
permissions as possible without denying any permissions that the NFSv4
acl in a richacl grants.

This algorithm is used when a file inherits an acl at create time and
when an acl is set via a mechanism that does not specify file modes
(such as via nfsd).  When user-space sets an acl, the file masks are
passed in as part of the xattr.

When setting a richacl, the file masks determine what the file
permission bits will be set to; see richacl_masks_to_mode().

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: Update the file masks in chmod()

Doing a chmod() sets the file mode, which includes the file permission
bits.  When a file has a richacl, the permissions that the richacl
grants need to be limited to what the new file permission bits allow.

This is done by setting the file masks in the richacl to what the file
permission bits map to.  The richacl access check algorithm takes the
file masks into account, which ensures that the richacl cannot grant too
many permissions.

It is possible to explicitly add permissions to the file masks which go
beyond what the file permission bits can grant (like the ACE4_WRITE_ACL
permission).  The POSIX.1 standard calls this an alternate file access
control mechanism.  A subsequent chmod() would ensure that those
permissions are disabled again.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: Permission check algorithm

As in the standard POSIX file permission model, each process is the
owner, group, or other file class.  A process is

  - in the owner file class if it owns the file,
  - in the group file class if it is in the file's owning group or it
    matches any of the user or group entries, and
  - in the other file class otherwise.

Each file class is associated with a file mask.

A richacl grants a requested access if the NFSv4 acl in the richacl
grants the requested permissions (according to the NFSv4 permission
check algorithm) and the file mask that applies to the process includes
the requested permissions.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: Create-time inheritance

When a new file is created, it can inherit an acl from its parent
directory; this is similar to how default acls work in POSIX (draft)
ACLs.

As with POSIX ACLs, if a file inherits an acl from its parent directory,
the intersection between the create mode and the permissions granted by
the inherited acl determines the file masks and file permission bits,
and the umask is ignored.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: Check if an acl is equivalent to a file mode

This function is used to avoid storing richacls on disk if the acl can
be computed from the file permission bits.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: Automatic Inheritance

Automatic Inheritance (AI) allows changes to the acl of a directory to
recursively propagate down to files and directories in the directory.

To implement this, the kernel keeps track of which permissions have been
inherited, and makes sure that permission propagation is turned off when
the file permission bits of a file are changed (upon create or chmod).

The actual permission propagation is implemented in user space.

AI works as follows:

 - When the ACL4_AUTO_INHERIT flag in the acl of a file is cleared, the
   file is not affected by AI.

 - When the ACL4_AUTO_INHERIT flag in the acl of a directory is set and
   a file or subdirectory is created in that directory, files created in
   the directory will have the ACL4_AUTO_INHERIT flag set, and all
   inherited aces will have the ACE4_INHERITED_ACE flag set.  This
   allows user space to distinguish between aces which have been
   inherited, and aces which have been explicitly added.

 - When the ACL4_PROTECTED acl flag in the acl of a file is set, AI will
   not modify the acl of the file.  This does not affect propagation of
   permissions from the file to its children (if the file is a
   directory).

Linux does not have a way of creating files without setting the file
permission bits, so all files created inside a directory with
ACL4_AUTO_INHERIT set will also have the ACL4_PROTECTED flag set.  This
effectively disables AI.

Protocols which support creating files without specifying permissions
can explicitly clear the ACL4_PROTECTED flag after creating a file (and
reset the file masks to "undo" applying the create mode; see
richacl_compute_max_masks()).  This is a workaround; a per-create or
per-process flag indicating to ignore the create mode when AI is in
effect would fix this problem.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
richacl: xattr mapping functions

Map between "system.richacl" xattrs and the in-kernel representation.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
vfs: Cache richacl in struct inode

Cache richacls in struct inode so that this doesn't have to be done
individually in each filesystem.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
vfs: Add richacl permission check

FIXME!!! need more explanation ?

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
ext4: Implement rich acl for ext4

Support the richacl permission model in ext4.  The richacls are stored
in "system.richacl" xattrs.This need to be enabled by tune2fs or during
mkfs.ext4

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
ext4: Add Ext4 compat richacl feature flag

This feature flag can be used to enable richacl on
the file system. Once enabled the "acl" mount option
will enable richacl instead of posix acl

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>