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 16:35:03 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Stanislav Fomichev <sdf@...gle.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 Wed, Aug 03, 2022 at 04:24:49PM -0700, Stanislav Fomichev wrote:
> On Wed, Aug 3, 2022 at 4:19 PM Martin KaFai Lau <kafai@...com> wrote:
> >
> > On Wed, Aug 03, 2022 at 03:59:26PM -0700, sdf@...gle.com wrote:
> > > 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.
> > Make sense.  I will add a comment on the requirement that the bpf prog type
> > needs to setup the bpf_run_ctx.
> 
> Thanks! White at it, is it worth adding a short sentence to
> sockopt_lock_sock on why it's safe to skip locking in the bpf case as
> well?
Yep. will do.

> Feels like the current state where bpf always runs with the locked
> socket might change in the future.
That likely will be from the sleepable bpf prog.
It can probably either acquire the lock in __bpf_setsockopt() before
calling sk_setsockopt() or flag the bpf_run_ctx to say the lock is not acquired.
The former should be more straight forward.