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