audit: Audit proc/<pid>/cmdline aka proctitle

During an audit event, cache and print the value of the process's
proctitle value (proc/<pid>/cmdline). This is useful in situations
where processes are started via fork'd virtual machines where the
comm field is incorrect. Often times, setting the comm field still
is insufficient as the comm width is not very wide and most
virtual machine "package names" do not fit. Also, during execution,
many threads have their comm field set as well. By tying it back to
the global cmdline value for the process, audit records will be more
complete in systems with these properties. An example of where this
is useful and applicable is in the realm of Android. With Android,
their is no fork/exec for VM instances. The bare, preloaded Dalvik
VM listens for a fork and specialize request. When this request comes
in, the VM forks, and the loads the specific application (specializing).
This was done to take advantage of COW and to not require a load of
basic packages by the VM on very app spawn. When this spawn occurs,
the package name is set via setproctitle() and shows up in procfs.
Many of these package names are longer then 16 bytes, the historical
width of task->comm. Having the cmdline in the audit records will
couple the application back to the record directly. Also, on my
Debian development box, some audit records were more useful then
what was printed under comm.

The cached proctitle is tied to the life-cycle of the audit_context
structure and is built on demand.

Proctitle is controllable by userspace, and thus should not be trusted.
It is meant as an aid to assist in debugging. The proctitle event is
emitted during syscall audits, and can be filtered with auditctl.

Example:
type=AVC msg=audit(1391217013.924:386): avc:  denied  { getattr } for  pid=1971 comm="mkdir" name="/" dev="selinuxfs" ino=1 scontext=system_u:system_r:consolekit_t:s0-s0:c0.c255 tcontext=system_u:object_r:security_t:s0 tclass=filesystem
type=SYSCALL msg=audit(1391217013.924:386): arch=c000003e syscall=137 success=yes exit=0 a0=7f019dfc8bd7 a1=7fffa6aed2c0 a2=fffffffffff4bd25 a3=7fffa6aed050 items=0 ppid=1967 pid=1971 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=4294967295 comm="mkdir" exe="/bin/mkdir" subj=system_u:system_r:consolekit_t:s0-s0:c0.c255 key=(null)
type=UNKNOWN[1327] msg=audit(1391217013.924:386):  proctitle=6D6B646972002D70002F7661722F72756E2F636F6E736F6C65

Acked-by: Steve Grubb <sgrubb@redhat.com> (wrt record formating)

Signed-off-by: William Roberts <wroberts@tresys.com>
Signed-off-by: Eric Paris <eparis@redhat.com>