From: Deepa Dinamani <redacted>
Date: Thu, 27 Dec 2018 18:55:09 -0800

Al Viro mentioned (Message-ID
[ref])
that there is probably a race condition
lurking in accesses of sk_stamp on 32-bit machines.

sock->sk_stamp is of type ktime_t which is always an s64.
On a 32 bit architecture, we might run into situations of
unsafe access as the access to the field becomes non atomic.

Use seqlocks for synchronization.
This allows us to avoid using spinlocks for readers as
readers do not need mutual exclusion.

Another approach to solve this is to require sk_lock for all
modifications of the timestamps. The current approach allows
for timestamps to have their own lock: sk_stamp_lock.
This allows for the patch to not compete with already
existing critical sections, and side effects are limited
to the paths in the patch.

The addition of the new field maintains the data locality
optimizations from
commit 9115e8cd2a0c ("net: reorganize struct sock for better data
locality")

Note that all the instances of the sk_stamp accesses
are either through the ioctl or the syscall recvmsg.

Signed-off-by: Deepa Dinamani <redacted>

Ok, I'm fine with this, so applied and queued up for -stable.

I will note in passing that there are several 32-bit architectures
that have 64-bit loads.  Sparc is one such case.  And they would not
need these changes.

But I don't think it's practical or worthwhile to add that level of
consideration into your changes.  I'd rather the commit stay as simple
as possible.

Thanks!