socket() with IPPROTO_ICMP uses UDP

[iAmGroute]

I compile the following to send ICMP packets:

#include <stdio.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#define ICMP_ECHO       8   /* Echo Request         */
int main() {
	struct sockaddr_in to;
	memset(&to, 0, sizeof(to));
	to.sin_family      = AF_INET;
	//to.sin_addr.s_addr = inet_addr("172.44.0.3");
	to.sin_addr.s_addr = inet_addr("1.1.1.1");
	to.sin_port        = 0;
	char packet[150] = { 0 };
	packet[0] = ICMP_ECHO;
	int fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_ICMP);
	while (1) {
		int r = sendto(fd, packet, sizeof(packet), 0, (struct sockaddr *)&to, sizeof(to));
		printf("%d %d\n", r, errno);
		sleep(4);
	}
}

with: gcc -static-pie -fPIC -Wall -o icmp_test icmp_test.c

• If I then run it on Linux 5.15.0-69-generic, it sends ICMP echo reply packets (as expected), as seen in tcpdump:

$ sudo tcpdump -nn -v -i wlp3s0 host 1.1.1.1
tcpdump: listening on wlp3s0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
17:26:30.999548 IP (tos 0x0, ttl 64, id 25840, offset 0, flags [DF], proto ICMP (1), length 170)
    ${my_ip} > 1.1.1.1: ICMP echo request, id 25, seq 0, length 150
...

• If I run it with app-elfloader:
  ../../run-app-elfloader/run_elfloader -r rootfs/ -k build/*_kvm-x86_64 -n icmp_test,
  it doesn't send out ICMP packets, but UDP.
  We can see this in the lwip debug output (proto=17 which is UDP, while ICMP would be 1):

[    6.461028] dbg:  [liblwip] <ip4.c @ 1097> IP header:
[    6.463983] dbg:  [liblwip] <ip4.c @ 1098> +-------------------------------+
[    6.467685] dbg:  [liblwip] <ip4.c @ 1099> | 4 | 5 |  0x00 |       178     | (v, hl, tos, len)
[    6.472054] dbg:  [liblwip] <ip4.c @ 1104> +-------------------------------+
[    6.475470] dbg:  [liblwip] <ip4.c @ 1105> |       22      |000|       0   | (id, flags, offset)
[    6.480519] dbg:  [liblwip] <ip4.c @ 1111> +-------------------------------+
[    6.484549] dbg:  [liblwip] <ip4.c @ 1112> |  255  |   17  |    0x0cf5     | (ttl, proto, chksum)
[    6.489074] dbg:  [liblwip] <ip4.c @ 1116> +-------------------------------+
[    6.493351] dbg:  [liblwip] <ip4.c @ 1117> |  172  |   44  |    0  |    2  | (src)
[    6.498078] dbg:  [liblwip] <ip4.c @ 1122> +-------------------------------+
[    6.501923] dbg:  [liblwip] <ip4.c @ 1123> |    1  |    1  |    1  |    1  | (dest)
[    6.505931] dbg:  [liblwip] <ip4.c @ 1128> +-------------------------------+

I can't see any references to IPPROTO_ICMP in this repo (when built), so I guess that's why.