linux-kernel - Re: [PATCH v9 perf,bpf 07/15] perf, bpf: save bpf_prog

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Date:   Wed, 13 Mar 2019 16:05:56 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        kernel-team@...com, peterz@...radead.org, acme@...hat.com,
        jolsa@...nel.org, namhyung@...nel.org, sdf@...ichev.me
Subject: Re: [PATCH v9 perf,bpf 07/15] perf, bpf: save bpf_prog_info
 information as headers to perf.data

Em Mon, Mar 11, 2019 at 10:30:43PM -0700, Song Liu escreveu:
> +static int write_bpf_prog_info(struct feat_fd *ff,
> +			       struct perf_evlist *evlist __maybe_unused)
> +{
> +	struct perf_env *env = &ff->ph->env;
> +	struct rb_root *root;
> +	struct rb_node *next;
> +	int ret;
> +
> +	down_read(&env->bpf_progs.lock);
> +
> +	ret = do_write(ff, &env->bpf_progs.infos_cnt,
> +		       sizeof(env->bpf_progs.infos_cnt));
> +	if (ret < 0)
> +		goto out;
> +
> +	root = &env->bpf_progs.infos;
> +	next = rb_first(root);
> +	while (next) {
> +		struct bpf_prog_info_node *node;
> +		size_t len;
> +
> +		node = rb_entry(next, struct bpf_prog_info_node, rb_node);
> +		next = rb_next(&node->rb_node);
> +		len = sizeof(struct bpf_prog_info_linear) +
> +			node->info_linear->data_len;
> +
> +		/* before writing to file, translate address to offset */
> +		bpf_program__bpil_addr_to_offs(node->info_linear);
> +		ret = do_write(ff, node->info_linear, len);
> +		/*
> +		 * translate back to address even when do_write() fails,
> +		 * so that this function never changes the data.
> +		 */
> +		bpf_program__bpil_offs_to_addr(node->info_linear);
> +		if (ret < 0)
> +			goto out;
> +	}
> +out:
> +	up_read(&env->bpf_progs.lock);
> +	return ret;
> +}

The above uses libbpf functions unconditionally, leading to build
failures when NO_LIBBPF=1 is passed or libbpf somehow is not available
or can't be built.

So I added this:

+#else // HAVE_LIBBPF_SUPPORT
+static int write_bpf_prog_info(struct feat_fd *ff __maybe_unused,
+                              struct perf_evlist *evlist __maybe_unused)
+{
+       return 0;
+}
+#endif // HAVE_LIBBPF_SUPPORT

And added this committer notes:

    Committer notes:
    
    We can't use the libbpf unconditionally, as the build may have been with
    NO_LIBBPF, when we end up with linking errors, so provide a dummy
    write_bpf_prog_info() wrapped by HAVE_LIBBPF_SUPPORT for that case.
    
    Reading and printing are not affected by this, so can continue as is.