perf record: Fix segfault when running with suid and kptr_restrict is 1

perf record: Fix segfault when running with suid and kptr_restrict is 1

Before this patch perf panics if kptr_restrict is set to 1 and perf is
owned by root with suid set:

  $ whoami
  wangnan
  $ ls -l ./perf
  -rwsr-xr-x 1 root root 19781908 Sep 21 19:29 /home/wangnan/perf
  $ cat /proc/sys/kernel/kptr_restrict
  1
  $ cat /proc/sys/kernel/perf_event_paranoid
  -1
  $ ./perf record -a
  Segmentation fault (core dumped)
  $

The reason is that perf assumes it is allowed to read kptr from
/proc/kallsyms when euid is root, but in fact the kernel doesn't allow
reading kptr when euid and uid do not match with each other:

  $ cp /bin/cat .
  $ sudo chown root:root ./cat
  $ sudo chmod u+s ./cat
  $ cat /proc/kallsyms | grep do_fork
  0000000000000000 T _do_fork          <--- kptr is hidden even euid is root
  $ sudo cat /proc/kallsyms | grep do_fork
  ffffffff81080230 T _do_fork

See lib/vsprintf.c for kernel side code.

This patch fixes this problem by checking both uid and euid.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/20161115040617.69788-3-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>