CAP(3)CAP(3)NAME
cap - capability for changing user name
SYNOPSIS
bind #¤ dir
dir/caphash
dir/capuse
DESCRIPTION
Cap allows a process owned by the host owner (see eve(10.2)) to give
another process on the same machine a capability to set its user name
to a specified user. The capability is a string of the form:
[ fromuser@ ] touser@key
where fromuser is a process's current user name, touser is its new user
name, and key is a string of random characters (eg, produced by secu‐
rity-random(2)).
Caphash is a write-only file that can only be opened by the host owner.
A process enables the use of a capability by writing the keyed hash of
fromuser@touser to caphash. The hash is computed using
Keyring->hmac_sha1 as follows:
kr := load Keyring Keyring->PATH;
IPint: import kr;
users := sys->sprint("%s@%s", fromuser, touser);
cap := sys->sprint("%s@%s", users, key);
digest := array[Keyring->SHA1dlen] of byte;
ausers := array of byte users;
kr->hmac_sha1(ausers, len ausers, array of byte key, digest, nil);
if(sys->write(caphashfd, digest, len digest) < 0)
error();
The capability (eg, cap in the example) can then be passed to another
process.
Capuse is a write-only file that can be opened by any process. It can
then write a capability string to change its user name, provided that
capability has previously been enabled by the host owner via caphash,
and if the capability included a fromuser, the writing process cur‐
rently has that user name. After a successful write, the writing
process will be owned by touser. Any capability can be used at most
once.
A capability enabled by caphash has a limited lifetime, on the order of
30 seconds. Caphash can be removed by the host owner to prevent its
further use.
SOURCE
/emu/port/devcap.c
/os/port/devcap.c
SEE ALSOkeyring-sha1(2), cons(3), intro(5), eve(10.2)
DIAGNOSTICS
A write to capuse without a previous write to caphash sets the error
string to ``invalid capability''.
CAP(3)