libselinux: resolv symlinks and dot directories before matching paths

matchpathcon cannot handle ./ or ../ in pathnames and doesn't do well
with symlinks.  This patch uses the glibc function realpath() to try to
determine a real path with resolved symlinks and dot directories.  For
example before this pach we would see:

$ matchpathcon /tmp/../eric
/tmp/../eric	<<none>>
$ matchpathcon /eric
/eric	system_u:object_r:default_t:s0

Whereas after the path we get the same results.  The one quirk with the
patch is that we need special code to make sure that realpath() does not
follow a symlink if it is the final component.  aka if we have a symlink
from /eric to /tmp/eric we do not want to resolv to /tmp/eric.  We want
to just resolv to the actual symlink /eric.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Dan Walsh <dwalsh@redhat.com>
1 file changed