mirror of https://github.com/ceph/ceph
113 lines
3.9 KiB
ReStructuredText
113 lines
3.9 KiB
ReStructuredText
======================
|
|
Capabilities in CephFS
|
|
======================
|
|
When a client wants to operate on an inode, it will query the MDS in various
|
|
ways, which will then grant the client a set of **capabilities**. These
|
|
grant the client permissions to operate on the inode in various ways. One
|
|
of the major differences from other network file systems (e.g NFS or SMB) is
|
|
that the capabilities granted are quite granular, and it's possible that
|
|
multiple clients can hold different capabilities on the same inodes.
|
|
|
|
Types of Capabilities
|
|
---------------------
|
|
There are several "generic" capability bits. These denote what sort of ability
|
|
the capability grants.
|
|
|
|
::
|
|
|
|
/* generic cap bits */
|
|
#define CEPH_CAP_GSHARED 1 /* client can reads (s) */
|
|
#define CEPH_CAP_GEXCL 2 /* client can read and update (x) */
|
|
#define CEPH_CAP_GCACHE 4 /* (file) client can cache reads (c) */
|
|
#define CEPH_CAP_GRD 8 /* (file) client can read (r) */
|
|
#define CEPH_CAP_GWR 16 /* (file) client can write (w) */
|
|
#define CEPH_CAP_GBUFFER 32 /* (file) client can buffer writes (b) */
|
|
#define CEPH_CAP_GWREXTEND 64 /* (file) client can extend EOF (a) */
|
|
#define CEPH_CAP_GLAZYIO 128 /* (file) client can perform lazy io (l) */
|
|
|
|
These are then shifted by a particular number of bits. These denote a part of
|
|
the inode's data or metadata on which the capability is being granted:
|
|
|
|
::
|
|
|
|
/* per-lock shift */
|
|
#define CEPH_CAP_SAUTH 2 /* A */
|
|
#define CEPH_CAP_SLINK 4 /* L */
|
|
#define CEPH_CAP_SXATTR 6 /* X */
|
|
#define CEPH_CAP_SFILE 8 /* F */
|
|
|
|
Only certain generic cap types are ever granted for some of those "shifts",
|
|
however. In particular, only the FILE shift ever has more than the first two
|
|
bits.
|
|
|
|
::
|
|
|
|
| AUTH | LINK | XATTR | FILE
|
|
2 4 6 8
|
|
|
|
From the above, we get a number of constants, that are generated by taking
|
|
each bit value and shifting to the correct bit in the word:
|
|
|
|
::
|
|
|
|
#define CEPH_CAP_AUTH_SHARED (CEPH_CAP_GSHARED << CEPH_CAP_SAUTH)
|
|
|
|
These bits can then be or'ed together to make a bitmask denoting a set of
|
|
capabilities.
|
|
|
|
There is one exception:
|
|
|
|
::
|
|
|
|
#define CEPH_CAP_PIN 1 /* no specific capabilities beyond the pin */
|
|
|
|
The "pin" just pins the inode into memory, without granting any other caps.
|
|
|
|
Graphically:
|
|
|
|
::
|
|
|
|
+---+---+---+---+---+---+---+---+
|
|
| p | _ |As x |Ls x |Xs x |
|
|
+---+---+---+---+---+---+---+---+
|
|
|Fs x c r w b a l |
|
|
+---+---+---+---+---+---+---+---+
|
|
|
|
The second bit is currently unused.
|
|
|
|
Abilities granted by each cap
|
|
-----------------------------
|
|
While that is how capabilities are granted (and communicated), the important
|
|
bit is what they actually allow the client to do:
|
|
|
|
* PIN: this just pins the inode into memory. This is sufficient to allow the
|
|
client to get to the inode number, as well as other immutable things like
|
|
major or minor numbers in a device inode, or symlink contents.
|
|
|
|
* AUTH: this grants the ability to get to the authentication-related metadata.
|
|
In particular, the owner, group and mode. Note that doing a full permission
|
|
check may require getting at ACLs as well, which are stored in xattrs.
|
|
|
|
* LINK: the link count of the inode
|
|
|
|
* XATTR: ability to access or manipulate xattrs. Note that since ACLs are
|
|
stored in xattrs, it's also sometimes necessary to access them when checking
|
|
permissions.
|
|
|
|
* FILE: this is the big one. These allow the client to access and manipulate
|
|
file data. It also covers certain metadata relating to file data -- the
|
|
size, mtime, atime and ctime, in particular.
|
|
|
|
Shorthand
|
|
---------
|
|
Note that the client logging can also present a compact representation of the
|
|
capabilities. For example:
|
|
|
|
::
|
|
|
|
pAsLsXsFs
|
|
|
|
The 'p' represents the pin. Each capital letter corresponds to the shift
|
|
values, and the lowercase letters after each shift are for the actual
|
|
capabilities granted in each shift.
|