sandbox incompatible with glibc 2.26 (openat() not handled for all our files)

If I enable the sandbox on my system, I get killed with:

(Sandbox) Caught a bad syscall attempt (syscall openat)
/usr/lib/x86_64-linux-gnu/libasan.so.4(+0x558c0)[0x7fb5203ab8c0]
/lib/x86_64-linux-gnu/libpthread.so.0(open64+0x4e)[0x7fb51ec6667e]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x13150)[0x7fb51ec67150]
/lib/x86_64-linux-gnu/libpthread.so.0(open64+0x4e)[0x7fb51ec6667e]
/usr/lib/x86_64-linux-gnu/libevent-2.1.so.6(evutil_open_closeonexec_+0x20)[0x7fb51fbb5540]
/usr/lib/x86_64-linux-gnu/libevent-2.1.so.6(evutil_read_file_+0x53)[0x7fb51fbb5603]
/usr/lib/x86_64-linux-gnu/libevent-2.1.so.6(evdns_base_load_hosts+0x8b)[0x7fb51fbc429b]
/home/dgoulet/Documents/git/tor/src/or/tor(+0x9f7adf)[0x55aa44446adf]
/home/dgoulet/Documents/git/tor/src/or/tor(do_main_loop+0x745)[0x55aa440f01d5]
/home/dgoulet/Documents/git/tor/src/or/tor(tor_run_main+0x1895)[0x55aa440f4065]
/home/dgoulet/Documents/git/tor/src/or/tor(tor_main+0x86)[0x55aa440e1fb6]
/home/dgoulet/Documents/git/tor/src/or/tor(main+0x1c)[0x55aa440df20c]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7fb51db741c1]
/home/dgoulet/Documents/git/tor/src/or/tor(_start+0x2a)[0x55aa440e1c6a]

strace output:

19360 openat(AT_FDCWD, "/etc/hosts", O_RDONLY|O_CLOEXEC) = 257
19360 --- SIGSYS {si_signo=SIGSYS, si_code=SYS_SECCOMP, si_call_addr=0x7f7d90ab667e, si_syscall=__NR_openat, si_arch=AUDIT_ARCH_X86_64 ---}

It is the first file being opened after the seccomp sandbox has been applied. Our sandbox code only considers "open()" to touch that file:

OPEN("/etc/hosts");

My libc is 2.26.

We probably need to handle the same files with openat() as we do with open() for this.