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Date:   Mon, 18 May 2020 16:55:21 -0700
From:   Andrii Nakryiko <>
To:     Qian Cai <>
Cc:     Alexei Starovoitov <>,
        Daniel Borkmann <>,
        Martin KaFai Lau <>,
        Song Liu <>, Yonghong Song <>,
        Andrii Nakryiko <>,
        John Fastabend <>,
        KP Singh <>,
        Linux Netdev List <>,
        bpf <>,
        Linux Kernel Mailing List <>,
        clang-built-linux <>
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 <> 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?

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