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]
