netdev - Re: [PATCH bpf-next 02/14] bpf: net: Avoid sock_setsockopt() taking sk lock when called from bpf

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Message-ID: <20220727200505.nzt7bxuc7yckjdcj@kafai-mbp.dhcp.thefacebook.com>
Date:   Wed, 27 Jul 2022 13:05:05 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "kernel-team@...com" <kernel-team@...com>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH bpf-next 02/14] bpf: net: Avoid sock_setsockopt() taking
 sk lock when called from bpf

On Wed, Jul 27, 2022 at 08:36:18AM +0000, David Laight wrote:
> From: Martin KaFai Lau
> > Sent: 27 July 2022 07:09
> > 
> > Most of the codes in bpf_setsockopt(SOL_SOCKET) are duplicated from
> > the sock_setsockopt().  The number of supported options are
> > increasing ever and so as the duplicated codes.
> > 
> > One issue in reusing sock_setsockopt() is that the bpf prog
> > has already acquired the sk lock.  sockptr_t is useful to handle this.
> > sockptr_t already has a bit 'is_kernel' to handle the kernel-or-user
> > memory copy.  This patch adds a 'is_bpf' bit to tell if sk locking
> > has already been ensured by the bpf prog.
> 
> That is a really horrid place to hide an 'is locked' bit.
> 
> You'd be better off splitting sock_setsockopt() to add a function
> that is called with sk_lock held and the value read.
> That would also save the churn of all the callers.
There is no churn to the callers after this patch, so quite
the opposite.