Re: [PATCH net 2/2] ipv4: Fix incorrect TOS in fibmatch route get reply

[Guillaume Nault <gnault@redhat.com>]

On Mon, Jul 15, 2024 at 05:23:54PM +0300, Ido Schimmel wrote:
> The TOS value that is returned to user space in the route get reply is
> the one with which the lookup was performed ('fl4->flowi4_tos'). This is
> fine when the matched route is configured with a TOS as it would not
> match if its TOS value did not match the one with which the lookup was
> performed.
> 
> However, matching on TOS is only performed when the route's TOS is not
> zero. It is therefore possible to have the kernel incorrectly return a
> non-zero TOS:
> 
>  # ip link add name dummy1 up type dummy
>  # ip address add 192.0.2.1/24 dev dummy1
>  # ip route get fibmatch 192.0.2.2 tos 0xfc
>  192.0.2.0/24 tos 0x1c dev dummy1 proto kernel scope link src 192.0.2.1
> 
> Fix by instead returning the DSCP field from the FIB result structure
> which was populated during the route lookup.
> 
> Output after the patch:
> 
>  # ip link add name dummy1 up type dummy
>  # ip address add 192.0.2.1/24 dev dummy1
>  # ip route get fibmatch 192.0.2.2 tos 0xfc
>  192.0.2.0/24 dev dummy1 proto kernel scope link src 192.0.2.1
> 
> Extend the existing selftests to not only verify that the correct route
> is returned, but that it is also returned with correct "tos" value (or
> without it).

Good catch!

Reviewed-by: Guillaume Nault <gnault@redhat.com>