netdev - Re: [PATCH v2 bpf-next 02/15] bpf: net: Avoid sk_setsockopt() taking sk lock when called from bpf

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Date:   Wed, 3 Aug 2022 15:59:26 -0700
From:   sdf@...gle.com
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf@...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,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH v2 bpf-next 02/15] bpf: net: Avoid sk_setsockopt() taking
 sk lock when called from bpf

On 08/03, Martin KaFai Lau wrote:
> Most of the code in bpf_setsockopt(SOL_SOCKET) are duplicated from
> the sk_setsockopt().  The number of supported optnames are
> increasing ever and so as the duplicated code.

> One issue in reusing sk_setsockopt() is that the bpf prog
> has already acquired the sk lock.  This patch adds a in_bpf()
> to tell if the sk_setsockopt() is called from a bpf prog.
> The bpf prog calling bpf_setsockopt() is either running in_task()
> or in_serving_softirq().  Both cases have the current->bpf_ctx
> initialized.  Thus, the in_bpf() only needs to test !!current->bpf_ctx.

> This patch also adds sockopt_{lock,release}_sock() helpers
> for sk_setsockopt() to use.  These helpers will test in_bpf()
> before acquiring/releasing the lock.  They are in EXPORT_SYMBOL
> for the ipv6 module to use in a latter patch.

> Note on the change in sock_setbindtodevice().  sockopt_lock_sock()
> is done in sock_setbindtodevice() instead of doing the lock_sock
> in sock_bindtoindex(..., lock_sk = true).

> Signed-off-by: Martin KaFai Lau <kafai@...com>
> ---
>   include/linux/bpf.h |  8 ++++++++
>   include/net/sock.h  |  3 +++
>   net/core/sock.c     | 26 +++++++++++++++++++++++---
>   3 files changed, 34 insertions(+), 3 deletions(-)

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 20c26aed7896..b905b1b34fe4 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1966,6 +1966,10 @@ static inline bool unprivileged_ebpf_enabled(void)
>   	return !sysctl_unprivileged_bpf_disabled;
>   }

> +static inline bool in_bpf(void)
> +{
> +	return !!current->bpf_ctx;
> +}

Good point on not needing to care about softirq!
That actually turned even nicer :-)

QQ: do we need to add a comment here about potential false-negatives?
I see you're adding ctx to the iter, but there is still a bunch of places
that don't use it.