[src/trunk]: src/sys/kern Make kern.arandom truncate the output instead of fa...

[nia <nia%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/db42082fd91a
branches:  trunk
changeset: 1009695:db42082fd91a
user:      nia <nia%NetBSD.org@localhost>
date:      Thu Apr 30 17:36:06 2020 +0000

description:
Make kern.arandom truncate the output instead of failing with ETOOBIG
when the requested data exceeds 256 bytes in size. The actual size of
the returned data is output to oldlenp.

This matches FreeBSD's behaviour and seems to be more in line with
what software in the wild expects.

"sounds reasonble" - Riastradh

diffstat:

 sys/kern/subr_cprng.c |  6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diffs (27 lines):

diff -r 930ec9014265 -r db42082fd91a sys/kern/subr_cprng.c
--- a/sys/kern/subr_cprng.c     Thu Apr 30 17:21:12 2020 +0000
+++ b/sys/kern/subr_cprng.c     Thu Apr 30 17:36:06 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: subr_cprng.c,v 1.36 2020/04/30 03:28:18 riastradh Exp $        */
+/*     $NetBSD: subr_cprng.c,v 1.37 2020/04/30 17:36:06 nia Exp $      */
 
 /*-
  * Copyright (c) 2019 The NetBSD Foundation, Inc.
@@ -52,7 +52,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_cprng.c,v 1.36 2020/04/30 03:28:18 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_cprng.c,v 1.37 2020/04/30 17:36:06 nia Exp $");
 
 #include <sys/types.h>
 #include <sys/cprng.h>
@@ -172,7 +172,7 @@
         * the past, so let's not break compatibility.
         */
        if (*oldlenp > 256)     /* size_t, so never negative */
-               return E2BIG;
+               *oldlenp = 256;
 
        /* Generate data.  */
        cprng_strong(user_cprng, buf, *oldlenp, 0);