netdev - Re: [PATCH v3] net: Add locking to protect skb->dev access in ip

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Message-ID: <CANn89i+srU_hJcfRbxcNg0cg02VkGmcPWCUV2k9vgh2+rzhU7A@mail.gmail.com>
Date: Wed, 30 Jul 2025 06:06:43 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Sharath Chandra Vurukala <quic_sharathv@...cinc.com>
Cc: davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, 
	pabeni@...hat.com, netdev@...r.kernel.org, quic_kapandey@...cinc.com, 
	quic_subashab@...cinc.com
Subject: Re: [PATCH v3] net: Add locking to protect skb->dev access in ip_output

On Wed, Jul 30, 2025 at 6:01 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Jul 30, 2025 at 3:51 AM Sharath Chandra Vurukala
> <quic_sharathv@...cinc.com> wrote:
> >
> > In ip_output() skb->dev is updated from the skb_dst(skb)->dev
> > this can become invalid when the interface is unregistered and freed,
> >
> > Introduced new skb_dst_dev_rcu() function to be used instead of
> > skb_dst_dev() within rcu_locks in ip_output.This will ensure that
> > all the skb's associated with the dev being deregistered will
> > be transnmitted out first, before freeing the dev.
> >
> > Given that ip_output() is called within an rcu_read_lock()
> > critical section or from a bottom-half context, it is safe to introduce
> > an RCU read-side critical section within it.
> >
> > Multiple panic call stacks were observed when UL traffic was run
> > in concurrency with device deregistration from different functions,
> > pasting one sample for reference.
> >
> > [496733.627565][T13385] Call trace:
> > [496733.627570][T13385] bpf_prog_ce7c9180c3b128ea_cgroupskb_egres+0x24c/0x7f0
> > [496733.627581][T13385] __cgroup_bpf_run_filter_skb+0x128/0x498
> > [496733.627595][T13385] ip_finish_output+0xa4/0xf4
> > [496733.627605][T13385] ip_output+0x100/0x1a0
> > [496733.627613][T13385] ip_send_skb+0x68/0x100
> > [496733.627618][T13385] udp_send_skb+0x1c4/0x384
> > [496733.627625][T13385] udp_sendmsg+0x7b0/0x898
> > [496733.627631][T13385] inet_sendmsg+0x5c/0x7c
> > [496733.627639][T13385] __sys_sendto+0x174/0x1e4
> > [496733.627647][T13385] __arm64_sys_sendto+0x28/0x3c
> > [496733.627653][T13385] invoke_syscall+0x58/0x11c
> > [496733.627662][T13385] el0_svc_common+0x88/0xf4
> > [496733.627669][T13385] do_el0_svc+0x2c/0xb0
> > [496733.627676][T13385] el0_svc+0x2c/0xa4
> > [496733.627683][T13385] el0t_64_sync_handler+0x68/0xb4
> > [496733.627689][T13385] el0t_64_sync+0x1a4/0x1a8
> >
> > Changes in v3:
> > - Replaced WARN_ON() with  WARN_ON_ONCE(), as suggested by Willem de Bruijn.
> > - Dropped legacy lines mistakenly pulled in from an outdated branch.
> >
> > Changes in v2:
> > - Addressed review comments from Eric Dumazet
> > - Used READ_ONCE() to prevent potential load/store tearing
> > - Added skb_dst_dev_rcu() and used along with rcu_read_lock() in ip_output
> >
> > Signed-off-by: Sharath Chandra Vurukala <quic_sharathv@...cinc.com>
>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Note: I have more patches coming, fixing IPv6 of course.