Re: [PATCH net-next] ipv4: Convert ->flowi4_tos to dscp_t.

[Ido Schimmel <idosch@nvidia.com>]

On Thu, Aug 21, 2025 at 01:32:03AM +0200, Guillaume Nault wrote:
> Convert the ->flowic_tos field of struct flowi_common from __u8 to
> dscp_t, rename it ->flowic_dscp and propagate these changes to struct
> flowi and struct flowi4.
> 
> We've had several bugs in the past where ECN bits could interfere with
> IPv4 routing, because these bits were not properly cleared when setting
> ->flowi4_tos. These bugs should be fixed now and the dscp_t type has
> been introduced to ensure that variables carrying DSCP values don't
> accidentally have any ECN bits set. Several variables and structure
> fields have been converted to dscp_t already, but the main IPv4 routing
> structure, struct flowi4, is still using a __u8. To avoid any future
> regression, this patch converts it to dscp_t.
> 
> There are many users to convert at once. Fortunately, around half of
> ->flowi4_tos users already have a dscp_t value at hand, which they
> currently convert to __u8 using inet_dscp_to_dsfield(). For all of
> these users, we just need to drop that conversion.
> 
> But, although we try to do the __u8 <-> dscp_t conversions at the
> boundaries of the network or of user space, some places still store
> TOS/DSCP variables as __u8 in core networking code. Some structure
> fields, like struct ip_tunnel_key::tos could be converted to dscp_t,
> but that would require a lot of work, so this is left for later. Other
> places can't be converted, for example because the data structure is
> part of UAPI or because the same variable or field is also used for
> handling ECN in other parts of the code. In all of these cases where we
> don't have a dscp_t variable at hand, we need to use
> inet_dsfield_to_dscp() when interacting with ->flowi4_dscp.
> 
> Signed-off-by: Guillaume Nault <gnault@redhat.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

Thanks! One nit below

[...]

> diff --git a/net/core/filter.c b/net/core/filter.c
> index da391e2b0788..c75f95c60af3 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2373,7 +2373,7 @@ static int __bpf_redirect_neigh_v4(struct sk_buff *skb, struct net_device *dev,
>  		struct flowi4 fl4 = {
>  			.flowi4_flags = FLOWI_FLAG_ANYSRC,
>  			.flowi4_mark  = skb->mark,
> -			.flowi4_tos   = inet_dscp_to_dsfield(ip4h_dscp(ip4h)),
> +			.flowi4_dscp   = ip4h_dscp(ip4h),

Nit: alignment is off

>  			.flowi4_oif   = dev->ifindex,
>  			.flowi4_proto = ip4h->protocol,
>  			.daddr	      = ip4h->daddr,