netdev - Re: [PATCH 1/6] bpf: Allow ctx access for pointers to scalar

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Message-ID: <CAADnVQJEkWT1VWW2h_ZO4UTMhfHuRzFjdXizwrbBG=tVf3Y9=w@mail.gmail.com>
Date:   Wed, 22 Jan 2020 17:16:20 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Yonghong Song <yhs@...com>, Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andriin@...com>,
        Martin Lau <kafai@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        David Miller <davem@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>
Subject: Re: [PATCH 1/6] bpf: Allow ctx access for pointers to scalar

On Wed, Jan 22, 2020 at 1:18 PM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Wed, Jan 22, 2020 at 08:09:59AM -0800, Alexei Starovoitov wrote:
>
> SNIP
>
> > > > > It cannot dereference it. Use it as what?
> > > >
> > > > If this is from original bcc code, it will use bpf_probe_read for
> > > > dereference. This is what I understand when I first reviewed this patch.
> > > > But it will be good to get Jiri's confirmation.
> > >
> > > it blocked me from accessing 'filename' argument when I probed
> > > do_sys_open via trampoline in bcc, like:
> > >
> > >     KRETFUNC_PROBE(do_sys_open)
> > >     {
> > >         const char *filename = (const char *) args[1];
> > >
> > > AFAICS the current code does not allow for trampoline arguments
> > > being other pointers than to void or struct, the patch should
> > > detect that the argument is pointer to scalar type and let it
> > > pass
> >
> > Got it. I've looked up your bcc patches and I agree that there is no way to
> > workaround. BTF type argument of that kernel function is 'const char *' and the
> > verifier will enforce that if bpf program tries to cast it the verifier will
> > still see 'const char *'. (It's done this way by design). How about we special
> > case 'char *' in the verifier? Then my concern regarding future extensibility
> > of 'int *' and 'long *' will go away.
> > Compilers have a long history special casing 'char *'. In particular signed
> > char because it's a pointer to null terminated string. I think it's still a
> > special pointer from pointer aliasing point of view. I think the verifier can
> > treat it as scalar here too. In the future the verifier will get smarter and
> > will recognize it as PTR_TO_NULL_STRING while 'u8 *', 'u32 *' will be
> > PTR_TO_BTF_ID. I think it will solve this particular issue. I like conservative
> > approach to the verifier improvements: start with strict checking and relax it
> > on case-by-case. Instead of accepting wide range of cases and cause potential
> > compatibility issues.
>
> ok, so something like below?
>
> jirka
>
>
> ---
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 832b5d7fd892..dd678b8e00b7 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3664,6 +3664,19 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
>         }
>  }
>
> +static bool is_string_ptr(struct btf *btf, const struct btf_type *t)
> +{
> +       /* t comes in already as a pointer */
> +       t = btf_type_by_id(btf, t->type);
> +
> +       /* allow const */
> +       if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
> +               t = btf_type_by_id(btf, t->type);
> +
> +       /* char, signed char, unsigned char */
> +       return btf_type_is_int(t) && t->size == 1;
> +}

yep. looks like btf doesn't distinguish signedness for chars.
So above is good.