selinux: add basic filtering for audit trace events

selinux: add basic filtering for audit trace events

This patch adds further attributes to the event. These attributes are
helpful to understand the context of the message and can be used
to filter the events.

There are three common items. Source context, target context and tclass.
There are also items from the outcome of operation performed.

An event is similar to:
           <...>-1309  [002] ....  6346.691689: selinux_audited:
       requested=0x4000000 denied=0x4000000 audited=0x4000000
       result=-13
       scontext=system_u:system_r:cupsd_t:s0-s0:c0.c1023
       tcontext=system_u:object_r:bin_t:s0 tclass=file

With systems where many denials are occurring, it is useful to apply a
filter. The filtering is a set of logic that is inserted with
the filter file. Example:
 echo "tclass==\"file\" " > events/avc/selinux_audited/filter

This adds that we only get tclass=file.

The trace can also have extra properties. Adding the user stack
can be done with
   echo 1 > options/userstacktrace

Now the output will be
         runcon-1365  [003] ....  6960.955530: selinux_audited:
     requested=0x4000000 denied=0x4000000 audited=0x4000000
     result=-13
     scontext=system_u:system_r:cupsd_t:s0-s0:c0.c1023
     tcontext=system_u:object_r:bin_t:s0 tclass=file
          runcon-1365  [003] ....  6960.955560: <user stack trace>
 =>  <00007f325b4ce45b>
 =>  <00005607093efa57>

Signed-off-by: Peter Enderborg <peter.enderborg@sony.com>
Reviewed-by: Thiébaud Weksteen <tweek@google.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>