netdev - Re: [PATCH] bpf: Forbid trampoline attach for functions with variable arguments

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Message-ID: <YJT2lbKUCRo90MQV@krava>
Date:   Fri, 7 May 2021 10:13:09 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH] bpf: Forbid trampoline attach for functions with
 variable arguments

On Fri, May 07, 2021 at 01:31:54AM +0200, Daniel Borkmann wrote:
> On 5/5/21 8:45 PM, Andrii Nakryiko wrote:
> > On Wed, May 5, 2021 at 6:42 AM Jiri Olsa <jolsa@...nel.org> wrote:
> > > 
> > > We can't currently allow to attach functions with variable arguments.
> > > The problem is that we should save all the registers for arguments,
> > > which is probably doable, but if caller uses more than 6 arguments,
> > > we need stack data, which will be wrong, because of the extra stack
> > > frame we do in bpf trampoline, so we could crash.
> > > 
> > > Also currently there's malformed trampoline code generated for such
> > > functions at the moment as described in:
> > >    https://lore.kernel.org/bpf/20210429212834.82621-1-jolsa@kernel.org/
> > > 
> > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > > ---
> > 
> > LGTM.
> > 
> > Acked-by: Andrii Nakryiko <andrii@...nel.org>
> > 
> > >   kernel/bpf/btf.c | 13 +++++++++++++
> > >   1 file changed, 13 insertions(+)
> > > 
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index 0600ed325fa0..161511bb3e51 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -5206,6 +5206,13 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
> > >          m->ret_size = ret;
> > > 
> > >          for (i = 0; i < nargs; i++) {
> > > +               if (i == nargs - 1 && args[i].type == 0) {
> > > +                       bpf_log(log,
> > > +                               "The function %s with variable args is unsupported.\n",
> > > +                               tname);
> > > +                       return -EINVAL;
> > > +
> 
> (Jiri, fyi, I removed this extra newline while applying. Please scan for such
> things before submitting.)

sorry, will do

jirka