exec: kill task_struct->did_exec

We can kill either task->did_exec or PF_FORKNOEXEC, they are mutually
exclusive. The patch kills ->did_exec because it has a single user.

Signed-off-by: Oleg Nesterov <[e-mail]>
Acked-by: KOSAKI Motohiro <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Kees Cook <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/exec.c

···
 		audit_bprm(bprm);
 		trace_sched_process_exec(current, old_pid, bprm);
 		ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
-		current->did_exec = 1;
 		proc_exec_connector(current);
 	}
 

include/linux/sched.h

···
 	/* Used for emulating ABI behavior of previous Linux versions */
 	unsigned int personality;
 
-	unsigned did_exec:1;
 	unsigned in_execve:1;	/* Tell the LSMs that the process is doing an
 				 * execve */
 	unsigned in_iowait:1;

kernel/fork.c

···
 	if (!try_module_get(task_thread_info(p)->exec_domain->module))
 		goto bad_fork_cleanup_count;
 
-	p->did_exec = 0;
 	delayacct_tsk_init(p);	/* Must remain after dup_task_struct() */
 	copy_flags(clone_flags, p);
 	INIT_LIST_HEAD(&p->children);

kernel/sys.c

···
  * only important on a multi-user system anyway, to make sure one user
  * can't send a signal to a process owned by another.  -TYT, 12/12/91
  *
- * Auch. Had to add the 'did_exec' flag to conform completely to POSIX.
- * LBT 04.03.94
+ * !PF_FORKNOEXEC check to conform completely to POSIX.
  */
 SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 {
···
 		if (task_session(p) != task_session(group_leader))
 			goto out;
 		err = -EACCES;
-		if (p->did_exec)
+		if (!(p->flags & PF_FORKNOEXEC))
 			goto out;
 	} else {
 		err = -ESRCH;