[PATCH v2] ipv6: default IPV6_SIT to m

[PATCH v2] ipv6: default IPV6_SIT to m

[Alyssa Ross]

This basically defaulted to m until recently, since IPV6 defaulted to
m.  Since IPV6 was changed to a boolean with a default of y, IPV6_SIT
started defaulting to built-in as well.  This results in a surprise
sit0 device by default for defconfig (and defconfig-derived config)
users at boot.  For me, this broke an (admittedly non-robust) script.
Preserve the behaviour of most configs by avoiding building this
module, that's probably overall seldom used compared to IPv6 as a
whole, into the kernel.

Fixes: 309b905deee59 ("ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs")
Signed-off-by: Alyssa Ross <hi@alyssa.is>
---
v2: update help text as well

 net/ipv6/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/Kconfig b/net/ipv6/Kconfig
index c024aa77f25ba..c3806c6ac96f9 100644
--- a/net/ipv6/Kconfig
+++ b/net/ipv6/Kconfig
@@ -164,7 +164,7 @@ config IPV6_SIT
 	select INET_TUNNEL
 	select NET_IP_TUNNEL
 	select IPV6_NDISC_NODETYPE
-	default y
+	default m
 	help
 	  Tunneling means encapsulating data of one protocol type within
 	  another protocol and sending it over a channel that understands the
@@ -172,7 +172,7 @@ config IPV6_SIT
 	  into IPv4 packets. This is useful if you want to connect two IPv6
 	  networks over an IPv4-only path.
 
-	  Saying M here will produce a module called sit. If unsure, say Y.
+	  Saying M here will produce a module called sit. If unsure, say M.
 
 config IPV6_SIT_6RD
 	bool "IPv6: IPv6 Rapid Deployment (6RD)"

base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
-- 
2.53.0

Re: [PATCH v2] ipv6: default IPV6_SIT to m

[Fernando Fernandez Mancera]

On 5/3/26 9:25 PM, Alyssa Ross wrote:
> This basically defaulted to m until recently, since IPV6 defaulted to
> m.  Since IPV6 was changed to a boolean with a default of y, IPV6_SIT
> started defaulting to built-in as well.  This results in a surprise
> sit0 device by default for defconfig (and defconfig-derived config)
> users at boot.  For me, this broke an (admittedly non-robust) script.
> Preserve the behaviour of most configs by avoiding building this
> module, that's probably overall seldom used compared to IPv6 as a
> whole, into the kernel.
> 
> Fixes: 309b905deee59 ("ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs")
> Signed-off-by: Alyssa Ross <hi@alyssa.is>

Hi, I am not sure about this. IPV6_SIT was already defaulting to 'y' for 
most distributions (except for ARM). Sure, if you had CONFIG_IPV6=m then 
CONFIG_IPV6_SIT was defaulting to 'm'.

Hm. Maybe we can aim this for net-next tree instead of net? I just don't 
think this is a fix but a change in behavior. FWIW, it makes sense to me 
to have this as a module by default.

Let's see what do others think.

Please respect the 24 hours period before reposting

https://docs.kernel.org/process/maintainer-netdev.html#resending-after-review

Thanks,
Fernando.

> ---
> v2: update help text as well
> 
>   net/ipv6/Kconfig | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv6/Kconfig b/net/ipv6/Kconfig
> index c024aa77f25ba..c3806c6ac96f9 100644
> --- a/net/ipv6/Kconfig
> +++ b/net/ipv6/Kconfig
> @@ -164,7 +164,7 @@ config IPV6_SIT
>   	select INET_TUNNEL
>   	select NET_IP_TUNNEL
>   	select IPV6_NDISC_NODETYPE
> -	default y
> +	default m
>   	help
>   	  Tunneling means encapsulating data of one protocol type within
>   	  another protocol and sending it over a channel that understands the
> @@ -172,7 +172,7 @@ config IPV6_SIT
>   	  into IPv4 packets. This is useful if you want to connect two IPv6
>   	  networks over an IPv4-only path.
>   
> -	  Saying M here will produce a module called sit. If unsure, say Y.
> +	  Saying M here will produce a module called sit. If unsure, say M.
>   
>   config IPV6_SIT_6RD
>   	bool "IPv6: IPv6 Rapid Deployment (6RD)"
> 
> base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731

Re: [PATCH v2] ipv6: default IPV6_SIT to m

[Alyssa Ross]

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Fernando Fernandez Mancera <fmancera@suse.de> writes:

> On 5/3/26 9:25 PM, Alyssa Ross wrote:
>> This basically defaulted to m until recently, since IPV6 defaulted to
>> m.  Since IPV6 was changed to a boolean with a default of y, IPV6_SIT
>> started defaulting to built-in as well.  This results in a surprise
>> sit0 device by default for defconfig (and defconfig-derived config)
>> users at boot.  For me, this broke an (admittedly non-robust) script.
>> Preserve the behaviour of most configs by avoiding building this
>> module, that's probably overall seldom used compared to IPv6 as a
>> whole, into the kernel.
>> 
>> Fixes: 309b905deee59 ("ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs")
>> Signed-off-by: Alyssa Ross <hi@alyssa.is>
>
> Hi, I am not sure about this. IPV6_SIT was already defaulting to 'y' for 
> most distributions (except for ARM). Sure, if you had CONFIG_IPV6=m then 
> CONFIG_IPV6_SIT was defaulting to 'm'.
>
> Hm. Maybe we can aim this for net-next tree instead of net? I just don't 
> think this is a fix but a change in behavior. FWIW, it makes sense to me 
> to have this as a module by default.
>
> Let's see what do others think.

Yes, it appears you're right.  I had only tested on ARM.  In that case,
perhaps it make sense to instead set this to m in the arm64 defconfig, to
preserve the previous situation, but I can also see the value in
reducing platform discongruity by letting it change to y on arm64 too.

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Re: [PATCH v2] ipv6: default IPV6_SIT to m

[Fernando Fernandez Mancera]

On 5/4/26 4:30 PM, Alyssa Ross wrote:
> Fernando Fernandez Mancera <fmancera@suse.de> writes:
> 
>> On 5/3/26 9:25 PM, Alyssa Ross wrote:
>>> This basically defaulted to m until recently, since IPV6 defaulted to
>>> m.  Since IPV6 was changed to a boolean with a default of y, IPV6_SIT
>>> started defaulting to built-in as well.  This results in a surprise
>>> sit0 device by default for defconfig (and defconfig-derived config)
>>> users at boot.  For me, this broke an (admittedly non-robust) script.
>>> Preserve the behaviour of most configs by avoiding building this
>>> module, that's probably overall seldom used compared to IPv6 as a
>>> whole, into the kernel.
>>>
>>> Fixes: 309b905deee59 ("ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs")
>>> Signed-off-by: Alyssa Ross <hi@alyssa.is>
>>
>> Hi, I am not sure about this. IPV6_SIT was already defaulting to 'y' for
>> most distributions (except for ARM). Sure, if you had CONFIG_IPV6=m then
>> CONFIG_IPV6_SIT was defaulting to 'm'.
>>
>> Hm. Maybe we can aim this for net-next tree instead of net? I just don't
>> think this is a fix but a change in behavior. FWIW, it makes sense to me
>> to have this as a module by default.
>>
>> Let's see what do others think.
> 
> Yes, it appears you're right.  I had only tested on ARM.  In that case,
> perhaps it make sense to instead set this to m in the arm64 defconfig, to
> preserve the previous situation, but I can also see the value in
> reducing platform discongruity by letting it change to y on arm64 too.

Changing it on arm64 defconfig would make sense to me too. We should 
probably visit other configs that might be in the same situation. That 
in my opinion would qualify a net tree fix as you proposed initially.

Re: [PATCH v2] ipv6: default IPV6_SIT to m

[Jakub Kicinski]

On Mon, 4 May 2026 17:01:43 +0200 Fernando Fernandez Mancera wrote:
> > Yes, it appears you're right.  I had only tested on ARM.  In that case,
> > perhaps it make sense to instead set this to m in the arm64 defconfig, to
> > preserve the previous situation, but I can also see the value in
> > reducing platform discongruity by letting it change to y on arm64 too.  
> 
> Changing it on arm64 defconfig would make sense to me too. We should 
> probably visit other configs that might be in the same situation. That 
> in my opinion would qualify a net tree fix as you proposed initially.

Right. I think the fix as posted makes sense. Tunnel drivers should 
be =m. If other arches default to =y that's likely an oversight.
Fedora and I suspect all other distros (and "prod kernels") set SIT=m.

Re: [PATCH v2] ipv6: default IPV6_SIT to m

[Fernando Fernandez Mancera]

On 5/5/26 2:09 AM, Jakub Kicinski wrote:
> On Mon, 4 May 2026 17:01:43 +0200 Fernando Fernandez Mancera wrote:
>>> Yes, it appears you're right.  I had only tested on ARM.  In that case,
>>> perhaps it make sense to instead set this to m in the arm64 defconfig, to
>>> preserve the previous situation, but I can also see the value in
>>> reducing platform discongruity by letting it change to y on arm64 too.
>>
>> Changing it on arm64 defconfig would make sense to me too. We should
>> probably visit other configs that might be in the same situation. That
>> in my opinion would qualify a net tree fix as you proposed initially.
> 
> Right. I think the fix as posted makes sense. Tunnel drivers should
> be =m. If other arches default to =y that's likely an oversight.
> Fedora and I suspect all other distros (and "prod kernels") set SIT=m.
> 

Yes. I checked and every distro I saw is setting this. This make sense, 
thanks Jakub.

Re: [PATCH v2] ipv6: default IPV6_SIT to m

[Fernando Fernandez Mancera]

On 5/3/26 9:25 PM, Alyssa Ross wrote:
> This basically defaulted to m until recently, since IPV6 defaulted to
> m.  Since IPV6 was changed to a boolean with a default of y, IPV6_SIT
> started defaulting to built-in as well.  This results in a surprise
> sit0 device by default for defconfig (and defconfig-derived config)
> users at boot.  For me, this broke an (admittedly non-robust) script.
> Preserve the behaviour of most configs by avoiding building this
> module, that's probably overall seldom used compared to IPv6 as a
> whole, into the kernel.
> 
> Fixes: 309b905deee59 ("ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs")
> Signed-off-by: Alyssa Ross <hi@alyssa.is>

As discussed:

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

Thanks!