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From:   Qian Cai <cai@lca.pw>
Date:   Sun, 17 May 2020 22:44:31 -0400
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Subject: UBSAN: array-index-out-of-bounds in kernel/bpf/arraymap.c:177
To:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Cc:     Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
        Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Linux Netdev List <netdev@vger.kernel.org>,
        bpf@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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With Clang 9.0.1,

return array->value + array->elem_size * (index & array->index_mask);

but array->value is,

char value[0] __aligned(8);

[  506.031548][ T4134] LTP: starting bpf_prog02
[  506.125326][ T4352]
================================================================================
[  506.134603][ T4352] UBSAN: array-index-out-of-bounds in
kernel/bpf/arraymap.c:177:22
[  506.142521][ T4352] index 8 is out of range for type 'char [0]'
[  506.148613][ T4352] CPU: 222 PID: 4352 Comm: bpf_prog02 Tainted: G
           L    5.7.0-rc5-next-20200515 #2
[  506.158632][ T4352] Hardware name: HPE Apollo 70
/C01_APACHE_MB         , BIOS L50_5.13_1.11 06/18/2019
[  506.169084][ T4352] Call trace:
[  506.172256][ T4352]  dump_backtrace+0x0/0x22c
[  506.176634][ T4352]  show_stack+0x28/0x34
[  506.180666][ T4352]  dump_stack+0x104/0x194
[  506.184877][ T4352]  __ubsan_handle_out_of_bounds+0xf0/0x120
[  506.190565][ T4352]  array_map_lookup_elem+0x90/0x94
[  506.195560][ T4352]  bpf_map_lookup_elem+0x48/0x60
[  506.200383][ T4352]  ___bpf_prog_run+0xe9c/0x2840
[  506.205109][ T4352]  __bpf_prog_run32+0x80/0xac
[  506.209673][ T4352]  __bpf_prog_run_save_cb+0x104/0x46c
[  506.214919][ T4352]  sk_filter_trim_cap+0x21c/0x2c4
[  506.219823][ T4352]  unix_dgram_sendmsg+0x45c/0x860
[  506.224725][ T4352]  sock_sendmsg+0x4c/0x74
[  506.228935][ T4352]  sock_write_iter+0x158/0x1a4
[  506.233584][ T4352]  __vfs_write+0x190/0x1d8
[  506.237874][ T4352]  vfs_write+0x13c/0x1b8
[  506.241992][ T4352]  ksys_write+0xb0/0x120
[  506.246108][ T4352]  __arm64_sys_write+0x54/0x88
[  506.250747][ T4352]  do_el0_svc+0x128/0x1dc
[  506.254957][ T4352]  el0_sync_handler+0xd0/0x268
[  506.259594][ T4352]  el0_sync+0x164/0x180
[  506.263747][ T4352]