netdev - Re: [PATCH RFC v3 bpf-next 2/4] bpf: Add bpf_copy_from

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Date:   Tue, 30 Jun 2020 11:23:07 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH RFC v3 bpf-next 2/4] bpf: Add bpf_copy_from_user() helper.

On Mon, Jun 29, 2020 at 5:28 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Jun 18, 2020 at 3:33 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> > > + *
> > > + * int bpf_copy_from_user(void *dst, u32 size, const void *user_ptr)
> >
> > Can we also add bpf_copy_str_from_user (or bpf_copy_from_user_str,
> > whichever makes more sense) as well?
>
> Those would have to wait. I think strings need better long term design.
> That would be separate patches.

I agree that it would be nice to have better support for strings, long
term, but that's beside the point.

I think bpf_copy_from_user_str() is a must have right now as a
sleepable counterpart to bpf_probe_read_user_str(), just like
bpf_copy_from_user() is a sleepable variant of bpf_probe_read_user().
Look at progs/strobemeta.h, it does bpf_probe_read_user_str() to get
user-space zero-terminated strings. It's well defined interface and
behavior. There is nothing extra needed beyond a sleepable variant of
bpf_probe_read_user_str() to allow Strobemeta reliably fetch data from
user-space from inside a sleepable BPF program.