Re: [PATCH net-next] rtnetlink: add RTNH_REJECT_MASK

On Sun, 28 Nov 2021 16:01:27 +0200
Ido Schimmel [off-list ref] wrote:

On Fri, Nov 26, 2021 at 04:43:11PM +0300, Alexander Mikhalitsyn wrote:

quoted

diff --git a/include/uapi/linux/rtnetlink.h b/include/uapi/linux/rtnetlink.h
index 5888492a5257..9c065e2fdef9 100644
--- a/include/uapi/linux/rtnetlink.h
+++ b/include/uapi/linux/rtnetlink.h

@@ -417,6 +417,9 @@ struct rtnexthop {
 #define RTNH_COMPARE_MASK	(RTNH_F_DEAD | RTNH_F_LINKDOWN | \
 				 RTNH_F_OFFLOAD | RTNH_F_TRAP)
 
+/* these flags can't be set by the userspace */
+#define RTNH_REJECT_MASK	(RTNH_F_DEAD | RTNH_F_LINKDOWN)
+
 /* Macros to handle hexthops */
 
 #define RTNH_ALIGNTO	4

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 4c0c33e4710d..805f5e05b56d 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c

@@ -685,7 +685,7 @@ static int fib_get_nhs(struct fib_info *fi, struct rtnexthop *rtnh,
 			return -EINVAL;
 		}
 
-		if (rtnh->rtnh_flags & (RTNH_F_DEAD | RTNH_F_LINKDOWN)) {
+		if (rtnh->rtnh_flags & RTNH_REJECT_MASK) {
 			NL_SET_ERR_MSG(extack,
 				       "Invalid flags for nexthop - can not contain DEAD or LINKDOWN");
 			return -EINVAL;

@@ -1363,7 +1363,7 @@ struct fib_info *fib_create_info(struct fib_config *cfg,
 		goto err_inval;
 	}
 
-	if (cfg->fc_flags & (RTNH_F_DEAD | RTNH_F_LINKDOWN)) {
+	if (cfg->fc_flags & RTNH_REJECT_MASK) {
 		NL_SET_ERR_MSG(extack,
 			       "Invalid rtm_flags - can not contain DEAD or LINKDOWN");

Instead of a deny list as in the legacy nexthop code, the new nexthop
code has an allow list (from rtm_to_nh_config()):

	if (nhm->nh_flags & ~NEXTHOP_VALID_USER_FLAGS) {
		NL_SET_ERR_MSG(extack, "Invalid nexthop flags in ancillary header");
		goto out;
	}

Where:

#define NEXTHOP_VALID_USER_FLAGS RTNH_F_ONLINK

So while the legacy nexthop code allows setting flags such as
RTNH_F_OFFLOAD, the new nexthop code denies them. I don't have a use
case for setting these flags from user space so I don't care if we allow
or deny them, but I believe the legacy and new nexthop code should be
consistent.

Dear Ido,

thanks for your attention to the patches and our checkpoint/restore problem.

Yep, I've read nexthop code too and notices some inconsistencies, but
unfortunately I'm newbie here and my first goal is to fix thing and not break
something, that's why my patch is so trivial and not invasive :)

We have some discussion about these flags here:
https://lore.kernel.org/netdev/d7c2d8fa-052e-b941-2ef1-830c1ba655c1@gmail.com/#r (local)

I've noticed, that current iproute2 code not allows us to set RTNH_F_OFFLOAD and
RTNH_F_TRAP directly. And asked If we should prohibit setting these flags from
the userspace. But huge thanks to Roopa and David here - it turned out that some
userspace code usings these flags and sets it.

So, let's decide which flags we should allow to set from the userspace side
and which not. I'm ready to prepare all needed changes for both the kernel and
iproute2 side. ;)

WDYT? Should we allow these flags in the new nexthop code as well or
keep denying them?

IMHO, we should try to be consistent between the new nexthop code and the lagacy one.

Regards,
Alex

quoted

 		goto err_inval;
-- 
2.31.1