linux-kernel - Re: [PATCH net-next v23 03/13] netdev: support binding dma-buf to netdevice

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Message-ID: <CAHS8izOy26r0uoWdASgmBCENNS6cDjHpkp+AHhOaKVkZR1LZqQ@mail.gmail.com>
Date: Thu, 29 Aug 2024 22:41:17 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
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Subject: Re: [PATCH net-next v23 03/13] netdev: support binding dma-buf to netdevice

On Thu, Aug 29, 2024 at 2:24 PM Vadim Fedorenko
<vadim.fedorenko@...ux.dev> wrote:
>
> On 29/08/2024 22:08, Jakub Kicinski wrote:
> > On Thu, 29 Aug 2024 06:01:16 +0000 Mina Almasry wrote:
> >> +    err = genlmsg_reply(rsp, info);
> >> +    if (err)
> >> +            goto err_unbind;
> >> +
> >>      return 0;
> >> +
> >> +err_unbind:
> >
> > rtnl_lock()
>
> There are 2 places with goto err_unbind, and one is under the lock,
> additional label (or rearrange of the last check) is needed..
>

Thank you, I think the right fix here is to reacquire rtnl_lock before
the `goto err_unbind;`, since err_unbind expects rtnl to be locked at
this point.

This could introduce a weird edge case where we drop rtnl_lock, then
find out genlmsg_reply failed, then reacquire rtnl_lock to do the
cleanup. I can't think of anything that would horribly break if we do
that, but I may be missing something. In theory we could race with a
dmabuf unbind call happening in parallel.

If we can't reacquire rtnl_lock to do the cleanup, I think I need to
revert back to doing genlmsg_reply inside of rtnl_lock, and dropping
the lock before we return from the function.

-- 
Thanks,
Mina