linux-kernel - Re: UBSAN: array-index-out-of-bounds in kernel/bpf/arraymap.c:177

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Date:   Mon, 18 May 2020 16:55:21 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Qian Cai <cai@....pw>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Linux Netdev List <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: UBSAN: array-index-out-of-bounds in kernel/bpf/arraymap.c:177

On Sun, May 17, 2020 at 7:45 PM Qian Cai <cai@....pw> wrote:
>
> With Clang 9.0.1,
>
> return array->value + array->elem_size * (index & array->index_mask);
>
> but array->value is,
>
> char value[0] __aligned(8);

This, and ptrs and pptrs, should be flexible arrays. But they are in a
union, and unions don't support flexible arrays. Putting each of them
into anonymous struct field also doesn't work:

/data/users/andriin/linux/include/linux/bpf.h:820:18: error: flexible
array member in a struct with no named members
   struct { void *ptrs[] __aligned(8); };

So it probably has to stay this way. Is there a way to silence UBSAN
for this particular case?