net: redirect nftables stdout and stderr to log CRIU's log file

[adrianreber]

When using the nftables network locking backend and restoring a process a second time the network locking has already been deleted by the first restore. The second restore will print out to the console text like:

Error: Could not process rule: No such file or directory
delete table inet CRIU-202621

With this change CRIU's log FD is used by linbftables stdout and stderr.

[rst0git]

The code in nftables_lock_network_internal() and nftables_network_unlock() looks very similar.

Would it make sense to create a function?

static inline int nftables_set_logfd(struct nft_ctx *nft)
{
	FILE *fp;
	fp = fdopen(log_get_fd(), "w");
	if (!fp) {
		pr_perror("fdopen() failed");
		return -1;
	}
	nft_ctx_set_output(nft, fp);
	nft_ctx_set_error(nft, fp);

	fflush(fp);
	fclose(fp);
	return 0;
}

We can use this function in nftables_lock_network_internal():

if (nftables_set_logfd(nft)) {
	goto err2;
}

and in nftables_network_unlock():

if (nftables_set_logfd(nft)) {
	nft_ctx_free(nft);
	return -1;
}

[avagin]

The nft_ctx_set_output() and nft_ctx_set_error() functions set the output or error file pointer in ctx to the value of fp. They return the previous value to aid in temporary file pointer overrides. On error, these functions return NULL. 

nftables_set_logfd can't close fp...