Re: [PATCH v9 bpf-next 3/5] bpf: Add skb dynptrs

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jan 31, 2023 at 9:55 AM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On Mon, Jan 30, 2023 at 9:36 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Mon, Jan 30, 2023 at 04:44:12PM -0800, Joanne Koong wrote:
> > > On Sun, Jan 29, 2023 at 3:39 PM Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > On Fri, Jan 27, 2023 at 11:17:01AM -0800, Joanne Koong wrote:
> > > > > Add skb dynptrs, which are dynptrs whose underlying pointer points
> > > > > to a skb. The dynptr acts on skb data. skb dynptrs have two main
> > > > > benefits. One is that they allow operations on sizes that are not
> > > > > statically known at compile-time (eg variable-sized accesses).
> > > > > Another is that parsing the packet data through dynptrs (instead of
> > > > > through direct access of skb->data and skb->data_end) can be more
> > > > > ergonomic and less brittle (eg does not need manual if checking for
> > > > > being within bounds of data_end).
> > > > >
> > > > > For bpf prog types that don't support writes on skb data, the dynptr is
> > > > > read-only (bpf_dynptr_write() will return an error and bpf_dynptr_data()
> > > > > will return a data slice that is read-only where any writes to it will
> > > > > be rejected by the verifier).
> > > > >
> > > > > For reads and writes through the bpf_dynptr_read() and bpf_dynptr_write()
> > > > > interfaces, reading and writing from/to data in the head as well as from/to
> > > > > non-linear paged buffers is supported. For data slices (through the
> > > > > bpf_dynptr_data() interface), if the data is in a paged buffer, the user
> > > > > must first call bpf_skb_pull_data() to pull the data into the linear
> > > > > portion.
> > > >
> > > > Looks like there is an assumption in parts of this patch that
> > > > linear part of skb is always writeable. That's not the case.
> > > > See if (ops->gen_prologue || env->seen_direct_write) in convert_ctx_accesses().
> > > > For TC progs it calls bpf_unclone_prologue() which adds hidden
> > > > bpf_skb_pull_data() in the beginning of the prog to make it writeable.
> > >
> > > I think we can make this assumption? For writable progs (referenced in
> > > the may_access_direct_pkt_data() function), all of them have a
> > > gen_prologue that unclones the buffer (eg tc_cls_act, lwt_xmit, sk_skb
> > > progs) or their linear portion is okay to write into by default (eg
> > > xdp, sk_msg, cg_sockopt progs).
> >
> > but the patch was preserving seen_direct_write in some cases.
> > I'm still confused.
>
> seen_direct_write is used to determine whether to actually unclone or
> not in the program's prologue function (eg tc_cls_act_prologue() ->
> bpf_unclone_prologue() where in bpf_unclone_prologue(), if
> direct_write was not true, then it can skip doing the actual
> uncloning).
>
> I think the part of the patch you're talking about regarding
> seen_direct_write is this in check_helper_call():
>
> + if (func_id == BPF_FUNC_dynptr_data &&
> +    dynptr_type == BPF_DYNPTR_TYPE_SKB) {
> +   bool seen_direct_write = env->seen_direct_write;
> +
> +   regs[BPF_REG_0].type |= DYNPTR_TYPE_SKB;
> +   if (!may_access_direct_pkt_data(env, NULL, BPF_WRITE))
> +     regs[BPF_REG_0].type |= MEM_RDONLY;
> +   else
> +     /*
> +     * Calling may_access_direct_pkt_data() will set
> +     * env->seen_direct_write to true if the skb is
> +     * writable. As an optimization, we can ignore
> +     * setting env->seen_direct_write.
> +     *
> +     * env->seen_direct_write is used by skb
> +     * programs to determine whether the skb's page
> +     * buffers should be cloned. Since data slice
> +     * writes would only be to the head, we can skip
> +     * this.
> +     */
> +     env->seen_direct_write = seen_direct_write;
> + }
>
> If the data slice for a skb dynptr is writable, then seen_direct_write
> gets set to true (done internally in may_access_direct_pkt_data()) so
> that the skb is actually uncloned, whereas if it's read-only, then
> env->seen_direct_write gets reset to its original value (since the
> may_access_direct_pkt_data() call will have set env->seen_direct_write
> to true)

I'm still confused.
When may_access_direct_pkt_data() returns false
it doesn't change seen_direct_write.
When it returns true it also sets seen_direct_write=true.
But the code above restores it to whatever value it had before.
How is this correct?
Are you saying that another may_access_direct_pkt_data() gets
called somewhere in the verifier that sets seen_direct_write=true?
But what's the harm in doing it twice or N times in all cases?