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I have confirmed that this implementation solves my problem. I don't know how helpful this strace -f log is for you, but it seems that we use close_range successfully.

[pid 280940] dup2(14, 100)              = 100
[pid 280940] dup2(13, 101)              = 101
[pid 280940] dup2(8, 102)               = 102
[pid 280940] dup2(7, 103)               = 103
[pid 280940] prlimit64(0, RLIMIT_CORE, {rlim_cur=0, rlim_max=0}, NULL) = 0
[pid 280940] rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
[pid 280940] close(14)                  = 0
[pid 280940] close(13)                  = 0
[pid 280940] openat(AT_FDCWD, "/dev/null", O_RDWR) = 13
[pid 280940] dup2(13, 0)                = 0
[pid 280940] dup2(9, 1)                 = 1
[pid 280940] dup2(10, 2)                = 2
[pid 280940] close(13)                  = 0
[pid 280940] uname({sysname="Linux", nodename="fedora", ...}) = 0
[pid 280940] prlimit64(0, RLIMIT_NOFILE, NULL, {rlim_cur=1024, rlim_max=512*1024}) = 0
[pid 280940] close_range(4, 99, 0)      = 0
[pid 280940] close_range(104, 1023, 0)  = 0

Attaching full strace log (notice I aborted the program a couple seconds in, since the issue arises early).

strace.debug.log

Also tested that my docker container successfully runs fuzzilli under root with the absurdly large RLIMIT_NOFILE.

Thanks a lot for providing this PR. Unfortunately, as it is a bit larger and wasn't really urgent for us, I didn't find a chance to review it, yet. I'll try to find some time for it.