From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Joanne Koong <joannelkoong@gmail.com>
Cc: Martin KaFai Lau <kafai@fb.com>, bpf <bpf@vger.kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <ast@kernel.org>
Subject: Re: [PATCH bpf-next v1 1/3] bpf: Add skb dynptrs
Date: Mon, 1 Aug 2022 15:14:14 -0700 [thread overview]
Message-ID: <CAEf4BzZHkVtibiROT=HP1AW1FQm0cJi7AM+CY5FUNnVWBXZkNA@mail.gmail.com> (raw)
In-Reply-To: <CAJnrk1ZCQ5nRB=jBUxPFyS4OhMvDX1t4ddFYX2LqkepMZg-12w@mail.gmail.com>
On Mon, Aug 1, 2022 at 2:16 PM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On Mon, Aug 1, 2022 at 12:38 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > On Mon, Aug 01, 2022 at 10:52:14AM -0700, Joanne Koong wrote:
> > > > Since we are on bpf_dynptr_write, what is the reason
> > > > on limiting it to the skb_headlen() ? Not implying one
> > > > way is better than another. would like to undertand the reason
> > > > behind it since it is not clear in the commit message.
> > > For bpf_dynptr_write, if we don't limit it to skb_headlen() then there
> > > may be writes that pull the skb, so any existing data slices to the
> > > skb must be invalidated. However, in the verifier we can't detect when
> > > the data slice should be invalidated vs. when it shouldn't (eg
> > > detecting when a write goes into the paged area vs when the write is
> > > only in the head). If the prog wants to write into the paged area, I
> > > think the only way it can work is if it pulls the data first with
> > > bpf_skb_pull_data before calling bpf_dynptr_write. I will add this to
> > > the commit message in v2
> > Note that current verifier unconditionally invalidates PTR_TO_PACKET
> > after bpf_skb_store_bytes(). Potentially the same could be done for
> > other new helper like bpf_dynptr_write(). I think this bpf_dynptr_write()
> > behavior cannot be changed later, so want to raise this possibility here
> > just in case it wasn't considered before.
>
> Thanks for raising this possibility. To me, it seems more intuitive
> from the user standpoint to have bpf_dynptr_write() on a paged area
> fail (even if bpf_dynptr_read() on that same offset succeeds) than to
> have bpf_dynptr_write() always invalidate all dynptr slices related to
> that skb. I think most writes will be to the data in the head area,
> which seems unfortunate that bpf_dynptr_writes to the head area would
> invalidate the dynptr slices regardless.
+1. Given bpf_skb_store_bytes() is a more powerful superset of
bpf_dynptr_write(), I'd keep bpf_dynptr_write() in such a form as to
play nicely with bpf_dynptr_data() pointers.
>
> What are your thoughts? Do you think you prefer having
> bpf_dynptr_write() always work regardless of where the data is? If so,
> I'm happy to make that change for v2 :)
>
> >
> > Thinking from the existing bpf_skb_{load,store}_bytes() and skb->data perspective.
> > If the user changes the skb by directly using skb->data to avoid calling
> > load_bytes()/store_bytes(), the user will do the necessary bpf_skb_pull_data()
> > before reading/writing the skb->data. If load_bytes()+store_bytes() is used instead,
> > it would be hard to reason why the earlier bpf_skb_load_bytes() can load a particular
> > byte but [may] need to make an extra bpf_skb_pull_data() call before it can use
> > bpf_skb_store_bytes() to store a modified byte at the same offset.
next prev parent reply other threads:[~2022-08-01 22:14 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-26 18:47 [PATCH bpf-next v1 0/3] Add skb + xdp dynptrs Joanne Koong
2022-07-26 18:47 ` [PATCH bpf-next v1 1/3] bpf: Add skb dynptrs Joanne Koong
2022-07-27 17:13 ` sdf
2022-07-28 16:49 ` Joanne Koong
2022-07-28 17:28 ` Stanislav Fomichev
2022-07-28 17:45 ` Hao Luo
2022-07-28 18:36 ` Joanne Koong
2022-07-28 23:39 ` Martin KaFai Lau
2022-07-29 20:26 ` Joanne Koong
2022-07-29 21:39 ` Martin KaFai Lau
2022-08-01 17:52 ` Joanne Koong
2022-08-01 19:38 ` Martin KaFai Lau
2022-08-01 21:16 ` Joanne Koong
2022-08-01 22:14 ` Andrii Nakryiko [this message]
2022-08-01 22:32 ` Martin KaFai Lau
2022-08-01 22:58 ` Andrii Nakryiko
2022-08-01 23:23 ` Martin KaFai Lau
2022-08-02 0:56 ` Martin KaFai Lau
2022-08-02 3:51 ` Andrii Nakryiko
2022-08-02 4:53 ` Joanne Koong
2022-08-02 5:14 ` Joanne Koong
2022-08-03 20:29 ` Joanne Koong
2022-08-03 20:36 ` Andrii Nakryiko
2022-08-03 20:56 ` Martin KaFai Lau
2022-08-03 23:25 ` Jakub Kicinski
2022-08-04 1:05 ` Joanne Koong
2022-08-04 1:34 ` Jakub Kicinski
2022-08-04 3:44 ` Joanne Koong
2022-08-04 1:27 ` Martin KaFai Lau
2022-08-04 1:44 ` Jakub Kicinski
2022-08-04 22:58 ` Kumar Kartikeya Dwivedi
2022-08-05 23:25 ` Jakub Kicinski
2022-08-01 22:11 ` Andrii Nakryiko
2022-08-02 0:15 ` Joanne Koong
2022-08-01 23:33 ` Jakub Kicinski
2022-08-02 2:12 ` Joanne Koong
2022-08-04 21:55 ` Joanne Koong
2022-08-05 23:22 ` Jakub Kicinski
2022-08-03 6:37 ` Martin KaFai Lau
2022-07-26 18:47 ` [PATCH bpf-next v1 2/3] bpf: Add xdp dynptrs Joanne Koong
2022-07-26 18:47 ` [PATCH bpf-next v1 3/3] selftests/bpf: tests for using dynptrs to parse skb and xdp buffers Joanne Koong
2022-07-26 19:44 ` Zvi Effron
2022-07-26 20:06 ` Joanne Koong
2022-08-01 17:58 ` Andrii Nakryiko
2022-08-02 22:56 ` Joanne Koong
2022-08-03 0:53 ` Andrii Nakryiko
2022-08-03 16:11 ` Joanne Koong
2022-08-04 18:45 ` Alexei Starovoitov
2022-08-05 16:29 ` Joanne Koong
2022-08-01 19:12 ` Alexei Starovoitov
2022-08-02 22:21 ` Joanne Koong
2022-08-04 21:46 ` Joanne Koong
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