[src/trunk]: src/sys/net bpf.c: support loopback writes when BIOCSHDRCMPLT is...

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

details:   https://anonhg.NetBSD.org/src/rev/af94e3eecec7
branches:  trunk
changeset: 373415:af94e3eecec7
user:      gutteridge <gutteridge%NetBSD.org@localhost>
date:      Wed Feb 08 01:37:53 2023 +0000

description:
bpf.c: support loopback writes when BIOCSHDRCMPLT is set

Following changes in r. 1.249 "bpf: support sending packets on loopback
interfaces", also allow for this to succeed when the "header complete"
flag is set, which is the practice of some tools, e.g., tcpreplay and
Scapy. With this change, both of those example tools now work, e.g.,
Scapy passes "L3bpfSocket - send and sniff on loopback" in its test
suite.

There are several ways of addressing this issue; this commit is
intended to be the most conservative and consistent with the previous
changes. (E.g., FreeBSD instead has special handling of this condition
in its if_loop.c.)

diffstat:

 sys/net/bpf.c |  11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diffs (32 lines):

diff -r f79b04892aca -r af94e3eecec7 sys/net/bpf.c
--- a/sys/net/bpf.c     Tue Feb 07 22:54:02 2023 +0000
+++ b/sys/net/bpf.c     Wed Feb 08 01:37:53 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: bpf.c,v 1.250 2023/02/07 01:46:37 gutteridge Exp $     */
+/*     $NetBSD: bpf.c,v 1.251 2023/02/08 01:37:53 gutteridge Exp $     */
 
 /*
  * Copyright (c) 1990, 1991, 1993
@@ -39,7 +39,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: bpf.c,v 1.250 2023/02/07 01:46:37 gutteridge Exp $");
+__KERNEL_RCSID(0, "$NetBSD: bpf.c,v 1.251 2023/02/08 01:37:53 gutteridge Exp $");
 
 #if defined(_KERNEL_OPT)
 #include "opt_bpf.h"
@@ -876,7 +876,12 @@
                goto out;
        }
 
-       if (d->bd_hdrcmplt)
+       /*
+        * If writing to a loopback interface, the address family has
+        * already been specially computed in bpf_movein(), so don't
+        * clobber it, or the loopback will reject it in looutput().
+        */
+       if (d->bd_hdrcmplt && ifp->if_type != IFT_LOOP)
                dst.ss_family = pseudo_AF_HDRCMPLT;
 
        if (d->bd_feedback) {