netdev - Re: [PATCH 12/14] bpf: Add trampolines to kallsyms

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Date:   Thu, 13 Feb 2020 00:02:32 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Andrii Nakryiko <andriin@...com>, Yonghong Song <yhs@...com>,
        Song Liu <songliubraving@...com>,
        Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH 12/14] bpf: Add trampolines to kallsyms

On Wed, Feb 12, 2020 at 08:33:49AM -0800, Andrii Nakryiko wrote:

SNIP

> > > >         tr->image = image;
> > > > +       INIT_LIST_HEAD_RCU(&tr->ksym.lnode);
> > > >  out:
> > > >         mutex_unlock(&trampoline_mutex);
> > > >         return tr;
> > > > @@ -267,6 +277,15 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(enum bpf_attach_type t)
> > > >         }
> > > >  }
> > > >
> > > > +static void bpf_trampoline_kallsyms_add(struct bpf_trampoline *tr)
> > > > +{
> > > > +       struct bpf_ksym *ksym = &tr->ksym;
> > > > +
> > > > +       snprintf(ksym->name, KSYM_NAME_LEN, "bpf_trampoline_%llu",
> > > > +                tr->key & ((u64) (1LU << 32) - 1));
> > >
> > > why the 32-bit truncation? also, wouldn't it be more trivial as (u32)tr->key?
> >
> > tr->key can have the target prog id in upper 32 bits,
> 
> True, but not clear why it's bad? It's not a security concern, because
> those IDs are already exposed (you can dump them from bpftool). On the
> other hand, by cutting out part of key, you make symbols potentially
> ambiguous, with different trampolines marked with the same name in
> kallsyms, which is just going to be confusing to users/tools.

ugh ok, I did not see the target bpf program case clearly,
will include the whole tr->key

thanks,
jirka