Re: [PATCH net 1/2] gre: Fix again IPv6 link-local address generation.

[Ido Schimmel <idosch@idosch.org>]

On Sat, May 03, 2025 at 12:57:52AM +0200, Guillaume Nault wrote:
> Use addrconf_addr_gen() to generate IPv6 link-local addresses on GRE
> devices in most cases and fall back to using add_v4_addrs() only in
> case the GRE configuration is incompatible with addrconf_addr_gen().
> 
> GRE used to use addrconf_addr_gen() until commit e5dd729460ca ("ip/ip6_gre:
> use the same logic as SIT interfaces when computing v6LL address")
> restricted this use to gretap and ip6gretap devices, and created
> add_v4_addrs() (borrowed from SIT) for non-Ethernet GRE ones.
> 
> The original problem came when commit 9af28511be10 ("addrconf: refuse
> isatap eui64 for INADDR_ANY") made __ipv6_isatap_ifid() fail when its
> addr parameter was 0. The commit says that this would create an invalid
> address, however, I couldn't find any RFC saying that the generated
> interface identifier would be wrong. Anyway, since gre over IPv4
> devices pass their local tunnel address to __ipv6_isatap_ifid(), that
> commit broke their IPv6 link-local address generation when the local
> address was unspecified.
> 
> Then commit e5dd729460ca ("ip/ip6_gre: use the same logic as SIT
> interfaces when computing v6LL address") tried to fix that case by
> defining add_v4_addrs() and calling it to generate the IPv6 link-local
> address instead of using addrconf_addr_gen() (apart for gretap and
> ip6gretap devices, which would still use the regular
> addrconf_addr_gen(), since they have a MAC address).
> 
> That broke several use cases because add_v4_addrs() isn't properly
> integrated into the rest of IPv6 Neighbor Discovery code. Several of
> these shortcomings have been fixed over time, but add_v4_addrs()
> remains broken on several aspects. In particular, it doesn't send any
> Router Sollicitations, so the SLAAC process doesn't start until the
> interface receives a Router Advertisement. Also, add_v4_addrs() mostly
> ignores the address generation mode of the interface
> (/proc/sys/net/ipv6/conf/*/addr_gen_mode), thus breaking the
> IN6_ADDR_GEN_MODE_RANDOM and IN6_ADDR_GEN_MODE_STABLE_PRIVACY cases.
> 
> Fix the situation by using add_v4_addrs() only in the specific scenario
> where the normal method would fail. That is, for interfaces that have
> all of the following characteristics:
> 
>   * run over IPv4,
>   * transport IP packets directly, not Ethernet (that is, not gretap
>     interfaces),
>   * tunnel endpoint is INADDR_ANY (that is, 0),
>   * device address generation mode is EUI64.
> 
> In all other cases, revert back to the regular addrconf_addr_gen().
> 
> Also, remove the special case for ip6gre interfaces in add_v4_addrs(),
> since ip6gre devices now always use addrconf_addr_gen() instead.
> 
> Note:
>   This patch was originally applied as commit 183185a18ff9 ("gre: Fix
>   IPv6 link-local address generation."). However, it was then reverted
>   by commit fc486c2d060f ("Revert "gre: Fix IPv6 link-local address
>   generation."") because it uncovered another bug that ended up
>   breaking net/forwarding/ip6gre_custom_multipath_hash.sh. That other
>   bug has now been fixed by commit 4d0ab3a6885e ("ipv6: Start path
>   selection from the first nexthop"). Therefore we can now revive this
>   GRE patch (no changes since original commit 183185a18ff9 ("gre: Fix
>   IPv6 link-local address generation.").
> 
> Fixes: e5dd729460ca ("ip/ip6_gre: use the same logic as SIT interfaces when computing v6LL address")
> Signed-off-by: Guillaume Nault <gnault@redhat.com>

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