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Message-ID: <63e561d8.a70a0220.250aa.3eb9@mx.google.com>
Date: Thu, 9 Feb 2023 13:12:55 -0800
From: Kees Cook <keescook@...omium.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
Alexei Starovoitov <ast@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>,
Shuah Khan <shuah@...nel.org>,
Haowen Bai <baihaowen@...zu.com>, bpf <bpf@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@...r.kernel.org>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Jesper Dangaard Brouer <hawk@...nel.org>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Tom Rix <trix@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>,
clang-built-linux <llvm@...ts.linux.dev>,
linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie_key
On Thu, Feb 09, 2023 at 12:50:28PM -0800, Alexei Starovoitov wrote:
> On Thu, Feb 9, 2023 at 12:05 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Thu, Feb 09, 2023 at 11:52:10AM -0800, Andrii Nakryiko wrote:
> > > Do we need to add a new type to UAPI at all here? We can make this new
> > > struct internal to kernel code (e.g. struct bpf_lpm_trie_key_kern) and
> > > point out that it should match the layout of struct bpf_lpm_trie_key.
> > > User-space can decide whether to use bpf_lpm_trie_key as-is, or if
> > > just to ensure their custom struct has the same layout (I see some
> > > internal users at Meta do just this, just make sure that they have
> > > __u32 prefixlen as first member).
> >
> > The uses outside the kernel seemed numerous enough to justify a new UAPI
> > struct (samples, selftests, etc). It also paves a single way forward
> > when the userspace projects start using modern compiler options (e.g.
> > systemd is usually pretty quick to adopt new features).
>
> I don't understand how the new uapi struct bpf_lpm_trie_key_u8 helps.
> cilium progs and progs/map_ptr_kern.c
> cannot do s/bpf_lpm_trie_key/bpf_lpm_trie_key_u8/.
> They will fail to build, so they're stuck with bpf_lpm_trie_key.
Right -- I'm proposing not changing bpf_lpm_trie_key. I'm proposing
_adding_ bpf_lpm_trie_key_u8 for new users who will be using modern
compiler options (i.e. where "data[0]" is nonsense).
> Can we do just
> struct bpf_lpm_trie_key_kern {
> __u32 prefixlen;
> __u8 data[];
> };
> and use it in the kernel?
Yeah, I can do that if that's preferred, but it leaves userspace hanging
when they eventually trip over this in their code when they enable
-fstrict-flex-arrays=3 too.
> What is the disadvantage?
It seemed better to give a working example of how to migrate this code.
Regardless, I can just make this specific to the kernel code if that's
what's wanted.
--
Kees Cook
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