[PATCH] libbpf: kprobe.multi: Filter with available_filter_functions_addrs

[PATCH] libbpf: kprobe.multi: Filter with available_filter_functions_addrs

[Jackie Liu]

From: Jackie Liu <liuyun01@kylinos.cn>

When using regular expression matching with "kprobe multi", it scans all
the functions under "/proc/kallsyms" that can be matched. However, not all
of them can be traced by kprobe.multi. If any one of the functions fails
to be traced, it will result in the failure of all functions. The best
approach is to filter out the functions that cannot be traced to ensure
proper tracking of the functions.

Use available_filter_functions_addrs check first, if failed, fallback to
kallsyms.

Here is the test eBPF program [1].
[1] https://github.com/JackieLiu1/ketones/tree/master/src/funccount

Suggested-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
---
 tools/lib/bpf/libbpf.c | 85 ++++++++++++++++++++++++++++++++++++++----
 1 file changed, 77 insertions(+), 8 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a27f6e9ccce7..3a114a5ac794 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -10107,6 +10107,12 @@ static const char *tracefs_uprobe_events(void)
 	return use_debugfs() ? DEBUGFS"/uprobe_events" : TRACEFS"/uprobe_events";
 }
 
+static const char *tracefs_available_filter_functions_addrs(void)
+{
+	return use_debugfs() ? DEBUGFS"/available_filter_functions_addrs" :
+			       TRACEFS"/available_filter_functions_addrs";
+}
+
 static void gen_kprobe_legacy_event_name(char *buf, size_t buf_sz,
 					 const char *kfunc_name, size_t offset)
 {
@@ -10422,9 +10428,8 @@ struct kprobe_multi_resolve {
 	size_t cnt;
 };
 
-static int
-resolve_kprobe_multi_cb(unsigned long long sym_addr, char sym_type,
-			const char *sym_name, void *ctx)
+static int ftrace_resolve_kprobe_multi_cb(unsigned long long sym_addr,
+					  const char *sym_name, void *ctx)
 {
 	struct kprobe_multi_resolve *res = ctx;
 	int err;
@@ -10441,6 +10446,63 @@ resolve_kprobe_multi_cb(unsigned long long sym_addr, char sym_type,
 	return 0;
 }
 
+static int
+kallsyms_resolve_kprobe_multi_cb(unsigned long long sym_addr, char sym_type,
+				 const char *sym_name, void *ctx)
+{
+	return ftrace_resolve_kprobe_multi_cb(sym_addr, sym_name, ctx);
+}
+
+typedef int (*available_kprobe_cb_t)(unsigned long long sym_addr,
+				     const char *sym_name, void *ctx);
+
+static int
+libbpf_available_kprobes_parse(available_kprobe_cb_t cb, void *ctx)
+{
+	unsigned long long sym_addr;
+	char sym_name[256];
+	FILE *f;
+	int ret, err = 0;
+	const char *available_path = tracefs_available_filter_functions_addrs();
+
+	f = fopen(available_path, "r");
+	if (!f) {
+		err = -errno;
+		pr_warn("failed to open %s, fallback to /proc/kallsyms.\n",
+			available_path);
+		return err;
+	}
+
+	while (true) {
+		ret = fscanf(f, "%llx %255s%*[^\n]\n", &sym_addr, sym_name);
+		if (ret == EOF && feof(f))
+			break;
+		if (ret != 2) {
+			pr_warn("failed to read available kprobe entry: %d\n",
+				ret);
+			err = -EINVAL;
+			break;
+		}
+
+		err = cb(sym_addr, sym_name, ctx);
+		if (err)
+			break;
+	}
+
+	fclose(f);
+	return err;
+}
+
+static void kprobe_multi_resolve_free(struct kprobe_multi_resolve *res)
+{
+	free(res->addrs);
+
+	/* reset to zero, when fallback */
+	res->cap = 0;
+	res->cnt = 0;
+	res->addrs = NULL;
+}
+
 struct bpf_link *
 bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 				      const char *pattern,
@@ -10477,9 +10539,16 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 		return libbpf_err_ptr(-EINVAL);
 
 	if (pattern) {
-		err = libbpf_kallsyms_parse(resolve_kprobe_multi_cb, &res);
-		if (err)
-			goto error;
+		err = libbpf_available_kprobes_parse(ftrace_resolve_kprobe_multi_cb,
+						     &res);
+		if (err) {
+			/* fallback to kallsyms */
+			kprobe_multi_resolve_free(&res);
+			err = libbpf_kallsyms_parse(kallsyms_resolve_kprobe_multi_cb,
+						    &res);
+			if (err)
+				goto error;
+		}
 		if (!res.cnt) {
 			err = -ENOENT;
 			goto error;
@@ -10512,12 +10581,12 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 		goto error;
 	}
 	link->fd = link_fd;
-	free(res.addrs);
+	kprobe_multi_resolve_free(&res);
 	return link;
 
 error:
 	free(link);
-	free(res.addrs);
+	kprobe_multi_resolve_free(&res);
 	return libbpf_err_ptr(err);
 }
 
-- 
2.25.1

Re: [PATCH] libbpf: kprobe.multi: Filter with available_filter_functions_addrs

[Jiri Olsa]

On Fri, Jun 23, 2023 at 10:12:45AM +0800, Jackie Liu wrote:

SNIP

> +
> +static void kprobe_multi_resolve_free(struct kprobe_multi_resolve *res)
> +{
> +	free(res->addrs);
> +
> +	/* reset to zero, when fallback */
> +	res->cap = 0;
> +	res->cnt = 0;
> +	res->addrs = NULL;
> +}
> +
>  struct bpf_link *
>  bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
>  				      const char *pattern,
> @@ -10477,9 +10539,16 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
>  		return libbpf_err_ptr(-EINVAL);
>  
>  	if (pattern) {
> -		err = libbpf_kallsyms_parse(resolve_kprobe_multi_cb, &res);
> -		if (err)
> -			goto error;
> +		err = libbpf_available_kprobes_parse(ftrace_resolve_kprobe_multi_cb,
> +						     &res);
> +		if (err) {
> +			/* fallback to kallsyms */
> +			kprobe_multi_resolve_free(&res);
> +			err = libbpf_kallsyms_parse(kallsyms_resolve_kprobe_multi_cb,
> +						    &res);
> +			if (err)
> +				goto error;
> +		}
>  		if (!res.cnt) {
>  			err = -ENOENT;
>  			goto error;
> @@ -10512,12 +10581,12 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
>  		goto error;
>  	}
>  	link->fd = link_fd;
> -	free(res.addrs);
> +	kprobe_multi_resolve_free(&res);

we don't need to zero res in here, so we could just leave the original
free(res.addrs) in here and perhaps rename kprobe_multi_resolve_free
to kprobe_multi_resolve_reinit

thanks,
jirka

>  	return link;
>  
>  error:
>  	free(link);
> -	free(res.addrs);
> +	kprobe_multi_resolve_free(&res);
>  	return libbpf_err_ptr(err);
>  }
>  
> -- 
> 2.25.1
>