Re: [PATCH v2 net-next] ipv4: Convert ->flowi4_tos to dscp_t.

[Ido Schimmel <idosch@nvidia.com>]

On Mon, Aug 25, 2025 at 03:37:43PM +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. Those can hardly be
> converted either 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.
> 
> Changes since v1:
>   * Fix space alignment in __bpf_redirect_neigh_v4() (Ido).
> 
> Signed-off-by: Guillaume Nault <gnault@redhat.com>

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