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