netdev - Re: [PATCH net-next v2 3/3] netlink: Fix wraparound of sk->sk_rmem

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Message-ID: <20250623163551.7973e198@kernel.org>
Date: Mon, 23 Jun 2025 16:35:51 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Kuniyuki Iwashima <kuni1840@...il.com>
Cc: jbaron@...mai.com, davem@...emloft.net, edumazet@...gle.com,
 horms@...nel.org, kuniyu@...gle.com, netdev@...r.kernel.org,
 pabeni@...hat.com
Subject: Re: [PATCH net-next v2 3/3] netlink: Fix wraparound of
 sk->sk_rmem_alloc

On Wed, 18 Jun 2025 23:13:02 -0700 Kuniyuki Iwashima wrote:
> From: Jason Baron <jbaron@...mai.com>
> Date: Wed, 18 Jun 2025 19:13:23 -0400
> > For netlink sockets, when comparing allocated rmem memory with the
> > rcvbuf limit, the comparison is done using signed values. This means
> > that if rcvbuf is near INT_MAX, then sk->sk_rmem_alloc may become
> > negative in the comparison with rcvbuf which will yield incorrect
> > results.
> > 
> > This can be reproduced by using the program from SOCK_DIAG(7) with
> > some slight modifications. First, setting sk->sk_rcvbuf to INT_MAX
> > using SO_RCVBUFFORCE and then secondly running the "send_query()"
> > in a loop while not calling "receive_responses()". In this case,
> > the value of sk->sk_rmem_alloc will continuously wrap around
> > and thus more memory is allocated than the sk->sk_rcvbuf limit.
> > This will eventually fill all of memory leading to an out of memory
> > condition with skbs filling up the slab.
> > 
> > Let's fix this in a similar manner to:
> > commit 5a465a0da13e ("udp: Fix multiple wraparounds of sk->sk_rmem_alloc.")
> > 
> > As noted in that fix, if there are multiple threads writing to a
> > netlink socket it's possible to slightly exceed rcvbuf value. But as
> > noted this avoids an expensive 'atomic_add_return()' for the common
> > case.  
> 
> This was because UDP RX path is the fast path, but netlink isn't.
> Also, it's common for UDP that multiple packets for the same socket
> are processed concurrently, and 850cbaddb52d dropped lock_sock from
> the path.

To be clear -- are you saying we should fix this differently?
Or perhaps that the problem doesn't exist? The change doesn't
seem very intrusive..