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diff --git a/doc/cephfs/capabilities.rst b/doc/cephfs/capabilities.rst new file mode 100644 index 000000000..ac47bb12b --- /dev/null +++ b/doc/cephfs/capabilities.rst @@ -0,0 +1,112 @@ +====================== +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. |