netdev - Re: [PATCH v4 bpf-next 1/9] bpf: Introduce bpf timers.

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Date:   Tue, 6 Jul 2021 18:38:06 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: Introduce bpf timers.

On Sun, Jul 4, 2021 at 7:19 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> > > +                     struct bpf_insn ld_addrs[2] = {
> > > +                             BPF_LD_IMM64(BPF_REG_3, (long)prog),
> > The "prog" pointer value is used here.
> >
> > > +                     };
> > > +
> > > +                     insn_buf[0] = ld_addrs[0];
> > > +                     insn_buf[1] = ld_addrs[1];
> > > +                     insn_buf[2] = *insn;
> > > +                     cnt = 3;
> > > +
> > > +                     new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> > > +                     if (!new_prog)
> > > +                             return -ENOMEM;
> > > +
> > > +                     delta    += cnt - 1;
> > > +                     env->prog = prog = new_prog;
> > After bpf_patch_insn_data(), a new prog may be allocated.
> > Is the above old "prog" pointer value updated accordingly?
> > I could have missed something.
>
> excellent catch. The patching of prog can go bad either here
> or later if patching of some other insn happened to change prog.
> I'll try to switch to dynamic prog fetching via ksym.
> The timers won't work in the interpreted mode though.
> But that's better trade-off than link-list of insns to patch with a prog
> after all of bpf_patch_insn_data are done?
> Some other way to fix this issue?

fyi we've discussed it with Martin offline and he came up with an excellent idea
to patch prog->aux instead of prog here. I'll add that to a respin.