Re: [PATCH v3 net-next 1/6] net: Order blackhole_netdev_init(), inet_init(), and inet6_init().

[Fernando Fernandez Mancera <fmancera@suse.de>]

On 10/1/26 10:47 PM, Kuniyuki Iwashima wrote:
> Commit 309b905deee5 ("ipv6: convert CONFIG_IPV6 to built-in
> only and clean up Kconfigs") started to call inet6_init() as
> device_initcall().
> 
> It explains the reason as a race condition on IPv6 addrconf,
> but this is not accurate.
> 
> The change initially used fs_initcall() but switched to
> device_initcall() due to the null-ptr-deref stack trace in
> the link below.
> 
> It happened because blackhole_netdev_init() uses device_initcall()
> and blackhole_netdev was NULL when addrconf_init() called
> ipv6_add_dev(blackhole_netdev).
> 
> Due to the order in Kbuild,
> 
>    $ cat -n Kbuild | grep -E "(drivers|net)"
>       111	obj-y			+= drivers/
>       114	obj-$(CONFIG_NET)	+= net/
> 
> using device_initcall() for IPv6 allows built-in drivers to
> be initialised earlier, which requires unnecessary NULL checks.
> 
> For the same ordering reason, IPv4 uses late_initcall() to
> initialise blackhole_netdev.
> 
> Given loopback_net_ops is registered in net_dev_init() at
> subsys_initcall(), blackhole_netdev_init() can be called
> after that.
> 
> In addition, some places assume that IPv4 must be initialised
> before IPv6.  For example, mptcp_proto_v6_init() copies
> mptcp_prot to mptcp_v6_prot, which would otherwise have NULL
> mptcp_v6_prot.h.hashinfo.
> 
> Let's explicitly order blackhole_netdev_init() -> inet_init()
> -> inet6_init() before device_initcall() with 3 different
> initcall levels:
> 
>    subsys_initcall      : 4  : net_dev_init()
>    subsys_initcall_sync : 4s : blackhole_netdev_init()
>    fs_initcall          : 5  : inet_init()
>    fs_initcall_sync     : 5s : inet6_init()
>    device_initcall      : 6  : built-in drivers
> 
> Note that both IPv4 and IPv6 can still use fs_initcall() thanks
> to the order in net/Makefile, but explicit ordering would be
> less error-prone.
> 
>    $ cat -n net/Makefile | grep ipv
>        17	obj-$(CONFIG_INET)		+= ipv4/
>        22	obj-y				+= ipv6/
> 
> Link: https://lore.kernel.org/netdev/20260309074758.0ea95a18@kernel.org/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>

Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>

I have tested this in multiple environments with multiple Kconfigs and 
command line options. Thanks for looking into this Kuniyuki.

> ---
> Cc: Fernando Fernandez Mancera <fmancera@suse.de>
> ---
>   drivers/net/loopback.c | 2 +-
>   net/ipv6/af_inet6.c    | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> index 1fb6ce6843ad..55ecd5ebac0f 100644
> --- a/drivers/net/loopback.c
> +++ b/drivers/net/loopback.c
> @@ -289,4 +289,4 @@ static int __init blackhole_netdev_init(void)
>   	return 0;
>   }
>   
> -device_initcall(blackhole_netdev_init);
> +subsys_initcall_sync(blackhole_netdev_init);
> diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
> index f0efdc13baf4..a9b8b4b42a1a 100644
> --- a/net/ipv6/af_inet6.c
> +++ b/net/ipv6/af_inet6.c
> @@ -1217,4 +1217,4 @@ static int __init inet6_init(void)
>   	proto_unregister(&tcpv6_prot);
>   	goto out;
>   }
> -device_initcall(inet6_init);
> +fs_initcall_sync(inet6_init);