netdev - Re: why is it not allowed to add a new socket protocol family as an external module?

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Date:	Thu, 21 Feb 2013 08:04:25 -0800
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Chris Friesen <chris.friesen@...band.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: why is it not allowed to add a new socket protocol family as an
 external module?

On Thu, 21 Feb 2013 09:47:33 -0600
Chris Friesen <chris.friesen@...band.com> wrote:

> On 02/20/2013 07:39 PM, Eric Dumazet wrote:
> > On Wed, 2013-02-20 at 18:44 -0600, Chris Friesen wrote:
> >
> >> Yes, there are a number of tables sized by NPROTO/AF_MAX/PF_MAX (and the
> >> fact that we use all three within the kernel is kind of sad) but there
> >> is no technical reason why we couldn't extend those dynamically if
> >> desired (with a linked list of additional protocols, perhaps).
> >>
> >> Hence my question--is the restriction for an ideological reason or
> >> simply because nobody thought it was worth the effort?
> >>
> >
> > I guess nobody did the preliminary work.
> >
> > lockdep might be the tricky part.
> >
> > net/core/sock.c:197:static const char *const af_family_key_strings[AF_MAX+1] = {
> > net/core/sock.c:211:  "sk_lock-AF_NFC"   , "sk_lock-AF_MAX"
> > net/core/sock.c:213:static const char *const af_family_slock_key_strings[AF_MAX+1] = {
> > net/core/sock.c:227:  "slock-AF_NFC"   , "slock-AF_MAX"
> > net/core/sock.c:229:static const char *const af_family_clock_key_strings[AF_MAX+1] = {
> > net/core/sock.c:243:  "clock-AF_NFC"   , "clock-AF_MAX"
> > net/core/sock.c:250:static struct lock_class_key af_callback_keys[AF_MAX];
> 
> Unless I'm missing something that looks straightforward.
> 
> When registering dynamically the new protocol would need to specify one 
> string, the protocol name.  ("AF_MYPROTOCOL" or something).  The three 
> key strings are derived from that.
> 
> The three lock_class_key structs (corresponding to 
> af_family_keys/af_family_slock_keys/af_callback_keys) can be 
> auto-allocated by the network core at dynamic registration time.
> 
> sock_lock_init(), sk_clone_lock(), and sock_init_data() would need 
> special-casing for protocol number larger than AF_MAX.  The most 
> readable would probably be to do the mapping from family to key/string 
> in helper functions rather than hard-coding a table offset right in 
> these routines.
> 
> Yes, there would be a runtime penalty, but it should be minimal.  For 
> static protocols it would be a few "sk->sk_family < AF_MAX" checks.  For 
> dynamic protocols they might need to walk a linked list, which in all 
> probability would be very short.

It is not impossible to make this dynamic, you would need to make the table an allocated
object and use proper locking like RCU. Oh, and because it is using GPL, the symbols
would have to be EXPORT_SYMBOL_GPL(), so any dream of proprietary stacks there would
be skating on even thinner ice. 
The lockdep stuff makes it more complicated but not impossible. 

The bigger issue is how would you manage statically assigned id's which
are not visible int headers or kernel source. How would you keep AF_VENDOR_PROTOCOL1 from
not colliding with AF_VENDOR_PROTOCOL2?
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