tomoyo: fallback to realpath if symlink's pathname does not exist

tomoyo: fallback to realpath if symlink's pathname does not exist

Alfred Agrell found that TOMOYO cannot handle execveat(AT_EMPTY_PATH)
inside chroot environment where /dev and /proc are not mounted, for
commit 51f39a1f0cea ("syscalls: implement execveat() system call") missed
that TOMOYO tries to canonicalize argv[0] when the filename fed to the
executed program as argv[0] is supplied using potentially nonexistent
pathname.

Since "/dev/fd/<fd>" already lost symlink information used for obtaining
that <fd>, it is too late to reconstruct symlink's pathname. Although
<filename> part of "/dev/fd/<fd>/<filename>" might not be canonicalized,
TOMOYO cannot use tomoyo_realpath_nofollow() when /dev or /proc is not
mounted. Therefore, fallback to tomoyo_realpath_from_path() when
tomoyo_realpath_nofollow() failed.

Reported-by: Alfred Agrell <blubban@gmail.com>
Closes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1082001
Fixes: 51f39a1f0cea ("syscalls: implement execveat() system call")
Cc: stable@vger.kernel.org # v3.19+
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>