Re: Breaking UAPI change?

[Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>]

Hello,

On Wed, 24 Mar 2021 16:30:58 +0100, Richard Weinberger wrote:
> Hi!
> 
> commit f5223e9eee65 ("can: extend sockaddr_can to include j1939 members") increased the size of 
> struct sockaddr_can.
> This is a problem for applications which use recvfrom() with addrlen being sizeof(struct sockaddr_can)
> or sizeof(struct sockaddr).
> If such an application was built before the change it will no longer function correctly on newer kernels.

This scenario was identified, and explicitely dealt with.
This requires a tiny bit different code, i.e. net/can/raw.c should use
REQUIRED_SIZE() instead of sizeof() for testing the size of the address
structure.

> In fact I ran into such a scenario and found the said commit later that day.

Looking to the v5.10 kernel (which I happen to have checked out),
your claim indeed seems true, the raw_recvmsg does not (raw_bind and
raw_sendmsg work correct, but that's not important for your problem).

> 
> Is this a known issue?

It wasn't, until you found it :-)

> Or is this allowed and application must not use sizeof(struct sockaddr_can) as addrlen?

sizeof(struct sockaddr_can). As you already mentioned, applications may have been built
before the size increase, and so they should not be recompiled.


> If so, what is the proposed way to avoid future breakage?

Your application should not change. Kernel must be fixed.

Kurt