exec: simplify initial stack size expansion

I had a hard time trying to understand completely why it is using vm_end in
one side of the expression and vm_start in the other one, and using
something in the "if" clause that is not an exact copy of what is used
below. The whole point is that the stack_size variable that was used in the
"if" clause is the difference between vm_start and vm_end, which is not far
away but makes this thing harder to read than it must be.
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/2017429.gqNitNVd0C@mobilepool36.emlix.com

fs/exec.c

@@ -843,16 +843,13 @@ int setup_arg_pages(struct linux_binprm *bprm,
 	 * will align it up.
 	 */
 	rlim_stack = bprm->rlim_stack.rlim_cur & PAGE_MASK;
+
+	stack_expand = min(rlim_stack, stack_size + stack_expand);
+
 #ifdef CONFIG_STACK_GROWSUP
-	if (stack_size + stack_expand > rlim_stack)
-		stack_base = vma->vm_start + rlim_stack;
-	else
-		stack_base = vma->vm_end + stack_expand;
+	stack_base = vma->vm_start + stack_expand;
 #else
-	if (stack_size + stack_expand > rlim_stack)
-		stack_base = vma->vm_end - rlim_stack;
-	else
-		stack_base = vma->vm_start - stack_expand;
+	stack_base = vma->vm_end - stack_expand;
 #endif
 	current->mm->start_stack = bprm->p;
 	ret = expand_stack(vma, stack_base);