exec: Factor bprm_stack_limits out of prepare_arg_pages

In preparation for implementiong kernel_execve (which will take kernel
pointers not userspace pointers) factor out bprm_stack_limits out of
prepare_arg_pages. This separates the counting which depends upon the
getting data from userspace from the calculations of the stack limits
which is usable in kernel_execve.

The remove prepare_args_pages and compute bprm->argc and bprm->envc
directly in do_execveat_common, before bprm_stack_limits is called.

Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: "Eric W. Biederman" <[e-mail]>

fs/exec.c

···
 	return i;
 }
 
-static int prepare_arg_pages(struct linux_binprm *bprm,
-			struct user_arg_ptr argv, struct user_arg_ptr envp)
+static int bprm_stack_limits(struct linux_binprm *bprm)
 {
 	unsigned long limit, ptr_size;
-
-	bprm->argc = count(argv, MAX_ARG_STRINGS);
-	if (bprm->argc < 0)
-		return bprm->argc;
-
-	bprm->envc = count(envp, MAX_ARG_STRINGS);
-	if (bprm->envc < 0)
-		return bprm->envc;
 
 	/*
 	 * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM
···
 		goto out_ret;
 	}
 
-	retval = prepare_arg_pages(bprm, argv, envp);
+	retval = count(argv, MAX_ARG_STRINGS);
+	if (retval < 0)
+		goto out_free;
+	bprm->argc = retval;
+
+	retval = count(envp, MAX_ARG_STRINGS);
+	if (retval < 0)
+		goto out_free;
+	bprm->envc = retval;
+
+	retval = bprm_stack_limits(bprm);
 	if (retval < 0)
 		goto out_free;