curl 7.77.0 (x86_64-pc-linux-gnu) libcurl/7.77.0 
Release-Date: 2021-05-26

Linux #1 SMP Mon Apr 26 20:57:08 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

#include <fcntl.h>
#include <unistd.h>
#include <curl/curl.h>

static int opensocket_cb(void *clientp, curlsocktype purpose, struct curl_sockaddr *address) {
    int fd;
    int relocated;

    fd = socket(address->family, address->socktype, address->protocol);    
    printf("opensocket_cb: fd=%d\n", fd);
    relocated = fcntl(fd, F_DUPFD, FD_SETSIZE);  // Smaller than FD_SETSIZE works fine
    printf("opensocket_cb: relocated=%d\n", relocated);
    close(fd);

    return relocated;
}

int main() {
    CURL *curl;
    CURLcode res;

    curl = curl_easy_init();
    curl_easy_setopt(curl, CURLOPT_URL, "http://ifconfig.co/ip");
    curl_easy_setopt(curl, CURLOPT_OPENSOCKETFUNCTION, opensocket_cb);

    res = curl_easy_perform(curl);
    printf("res: %d\n", res);

    return 0;
}

> time ./breakcurl
opensocket_cb: fd=3
opensocket_cb: relocated=1024
54.240.197.234
res: 0

real    0m1.205s
user    0m0.004s
sys     0m0.000s

1985  06:04:21.249430 sendto(1024, "GET /ip HTTP/1.1\r\nHost: ifconfig"..., 52, MSG_NOSIGNAL, NULL, 0) = 52
1985  06:04:21.249505 poll([{fd=1024, events=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND}], 1, 0) = 0 (Timeout)
1985  06:04:21.249540 rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7fbbde128c90}, NULL, 8) = 0
1985  06:04:21.249576 poll([{fd=4, events=POLLIN}], 1, 1000) = 0 (Timeout)

3221  06:07:32.037018 sendto(1023, "GET /ip HTTP/1.1\r\nHost: ifconfig"..., 52, MSG_NOSIGNAL, NULL, 0) = 52
3221  06:07:32.037066 poll([{fd=1023, events=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND}], 1, 0) = 0 (Timeout)
3221  06:07:32.037096 rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7ff35927fc90}, NULL, 8) = 0
3221  06:07:32.037128 poll([{fd=1023, events=POLLIN}, {fd=4, events=POLLIN}], 2, 186) = 1 ([{fd=1023, revents=POLLIN}])