linux-kernel - Re: [PATCH v2 bpf-next 3/4] libbpf: BTF dumper support for typed data

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Date:   Thu, 21 Jan 2021 11:51:42 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alan Maguire <alan.maguire@...cle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Bill Wendling <morbo@...gle.com>,
        Shuah Khan <shuah@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 3/4] libbpf: BTF dumper support for typed data

On Wed, Jan 20, 2021 at 10:56 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Sun, Jan 17, 2021 at 2:22 PM Alan Maguire <alan.maguire@...cle.com> wrote:
> >
> > Add a BTF dumper for typed data, so that the user can dump a typed
> > version of the data provided.
> >
> > The API is
> >
> > int btf_dump__emit_type_data(struct btf_dump *d, __u32 id,
> >                              const struct btf_dump_emit_type_data_opts *opts,
> >                              void *data);
> >

Two more things I realized about this API overnight:

1. It's error-prone to specify only the pointer to data without
specifying the size. If user screws up and scecifies wrong type ID or
if BTF data is corrupted, then this API would start reading and
printing memory outside the bounds. I think it's much better to also
require user to specify the size and bail out with error if we reach
the end of the allowed memory area.

2. This API would be more useful if it also returns the amount of
"consumed" bytes. That way users can do more flexible and powerful
pretty-printing of raw data. So on success we'll have >= 0 number of
bytes used for dumping given BTF type, or <0 on error. WDYT?

> > ...where the id is the BTF id of the data pointed to by the "void *"
> > argument; for example the BTF id of "struct sk_buff" for a
> > "struct skb *" data pointer.  Options supported are
> >
> >  - a starting indent level (indent_lvl)
> >  - a set of boolean options to control dump display, similar to those
> >    used for BPF helper bpf_snprintf_btf().  Options are
> >         - compact : omit newlines and other indentation
> >         - noname: omit member names
> >         - zero: show zero-value members
> >
> > Default output format is identical to that dumped by bpf_snprintf_btf(),
> > for example a "struct sk_buff" representation would look like this:
> >
> > struct sk_buff){
> >  (union){
> >   (struct){
>
> Curious, these explicit anonymous (union) and (struct), is that
> preferred way for explicitness, or is it just because it makes
> implementation simpler and thus was chosen? I.e., if the goal was to
> mimic C-style data initialization, you'd just have plain .next = ...,
> .prev = ..., .dev = ..., .dev_scratch = ..., all on the same level. So
> just checking for myself.
>
> >    .next = (struct sk_buff *)0xffffffffffffffff,
> >    .prev = (struct sk_buff *)0xffffffffffffffff,
> >    (union){
> >     .dev = (struct net_device *)0xffffffffffffffff,
> >     .dev_scratch = (long unsigned int)18446744073709551615,
> >    },
> >   },
> > ...
> >
> > Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
> > ---
>

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