Optimization flags other than -O0 causing segfault on clang

[bket]

I'm currently testing sslh-1.21 on OpenBSD current (amd64), which has clang-8.0.1 as base compiler. When the optimization level is other than -O0, sslh segfaults:

$ egdb --args ./sslh-fork -v 4                         
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Reading symbols from ./sslh-fork...done.
(gdb) r
Starting program: /home/code/sslh/sslh-fork -v 4

Program received signal SIGSEGV, Segmentation fault.
0x0000089a3c9b55b0 in config_setting_lookup () from /usr/local/lib/libconfig.so.11.0
(gdb) bt full
#0  0x0000089a3c9b55b0 in config_setting_lookup () from /usr/local/lib/libconfig.so.11.0
No symbol table info available.
#1  0x000008977e077b29 in config_setting_lookup_ud (cfg=0x4, desc=<optimized out>) at sslh-conf.c:1030
        name = "v"
        setting = <optimized out>
#2  0x000008977e078e77 in read_block_setval (target=0x8977e0880b0 <cfg>, cfg=0x4, desc=0x8977e0852c0 <table_sslhcfg>, errmsg=0x7f7ffffd01d8) at sslh-conf.c:1148
        setting = <optimized out>
        in_cfg = <error reading variable in_cfg (Cannot access memory at address 0x0)>
        i = <optimized out>
        block = <optimized out>
#3  0x000008977e0782f4 in read_block (cfg=0x4, target=0x8977e0880b0 <cfg>, desc=0x8977e0852c0 <table_sslhcfg>, errmsg=0x7f7ffffd01d8) at sslh-conf.c:1206
        set = <optimized out>
#4  0x000008977e07814f in sslhcfg_cl_parse (argc=<optimized out>, argv=<optimized out>, cfg=0x8977e0880b0 <cfg>) at sslh-conf.c:1501
        argtable = {0x89a3ad81b00, 0x899a75e1e00, 0x899c46e7000, 0x89a791fd680, 0x89a46e45100, 0x899a75e1180, 0x89a791fdc80, 0x899a75e1d80, 0x899c46e7d80, 
          0x899a75e1500, 0x89a46e45180, 0x899c46e7280, 0x899985c1700, 0x89a61d6da00, 0x89a61d6de00, 0x89a45087500, 0x89a3ad81f00, 0x89a61d6d000, 0x89a45087600, 
          0x89a3ad81a00, 0x89a45087900, 0x89a2960b400}
        nerrors = <optimized out>
        c = {root = 0x4, destructor = 0xb, options = 0, tab_width = 0, float_precision = 0, default_format = 5884, include_dir = 0x7f7ffffd0178 "", 
          include_fn = 0x89a1f8fc8c9 <_dl_find_symbol+457>, error_text = 0x7f7ffffd0188 "", error_file = 0x8977e06f338 "Y", error_line = 2114388485, 
          error_type = (CONFIG_ERR_FILE_IO | CONFIG_ERR_PARSE | unknown: 2196), filenames = 0x899cea50358, hook = 0x89a1da92400}
        errmsg = 0x7f7ffffd02a0 ""
        s = 0x40000000000000
        res = <optimized out>
#5  0x000008977e07b26a in main (argc=4, argv=0x7f7ffffd02b8, envp=<optimized out>) at sslh-main.c:268
        res = <optimized out>
        listen_sockets = <optimized out>
        num_addr_listen = <optimized out>

• Switching to GCC fixes this issue for all optimization levels
• Using -O0 with Clang fixes this issue

Any idea how to address this issue?