SQL> create or replace procedure p is
  2    user1 varchar2(30) := sys_context('userenv','current_user');
  3    user2 varchar2(30) := user;
  4  begin
  5    dbms_output.put_line('sys_context='||user1||', user='||user2)
  6  end;
  7  /

Procedure created.

SQL> exec p;
sys_context=MICHEL, user=MICHEL

PL/SQL procedure successfully completed.

SQL> grant execute on p to test;

Grant succeeded.

SQL> connect test/test;
Connected.
TEST> exec michel.p
sys_context=MICHEL, user=TEST

PL/SQL procedure successfully completed.