Re: [PATCH v6 RESEND] libbpf: kprobe.multi: Filter with available_filter_functions

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, May 26, 2023 at 11:50:26PM +0800, Jackie Liu wrote:
> 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 check first, if failed, fallback to
> kallsyms.
> 
> Here is the test eBPF program [1].
> [1] https://github.com/JackieLiu1/ketones/commit/a9e76d1ba57390e533b8b3eadde97f7a4535e867
> 
> Suggested-by: Jiri Olsa <olsajiri@gmail.com>
> Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
> ---
>  tools/lib/bpf/libbpf.c | 100 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 93 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index ad1ec893b41b..0914b7e98e30 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10106,6 +10106,12 @@ static const char *tracefs_uprobe_events(void)
>  	return use_debugfs() ? DEBUGFS"/uprobe_events" : TRACEFS"/uprobe_events";
>  }
>  
> +static const char *tracefs_available_filter_functions(void)
> +{
> +	return use_debugfs() ? DEBUGFS"/available_filter_functions" :
> +			       TRACEFS"/available_filter_functions";
> +}
> +
>  static void gen_kprobe_legacy_event_name(char *buf, size_t buf_sz,
>  					 const char *kfunc_name, size_t offset)
>  {
> @@ -10417,13 +10423,14 @@ static bool glob_match(const char *str, const char *pat)
>  struct kprobe_multi_resolve {
>  	const char *pattern;
>  	unsigned long *addrs;
> +	const char **syms;
>  	size_t cap;
>  	size_t cnt;
>  };
>  
>  static int
> -resolve_kprobe_multi_cb(unsigned long long sym_addr, char sym_type,
> -			const char *sym_name, void *ctx)
> +kallsyms_resolve_kprobe_multi_cb(unsigned long long sym_addr, char sym_type,
> +				 const char *sym_name, void *ctx)
>  {
>  	struct kprobe_multi_resolve *res = ctx;
>  	int err;
> @@ -10440,6 +10447,77 @@ resolve_kprobe_multi_cb(unsigned long long sym_addr, char sym_type,
>  	return 0;
>  }
>  
> +static int ftrace_resolve_kprobe_multi_cb(const char *sym_name, void *ctx)
> +{
> +	struct kprobe_multi_resolve *res = ctx;
> +	int err;
> +	char *name;
> +
> +	if (!glob_match(sym_name, res->pattern))
> +		return 0;
> +
> +	err = libbpf_ensure_mem((void **) &res->syms, &res->cap,
> +				sizeof(const char *), res->cnt + 1);
> +	if (err)
> +		return err;
> +
> +	name = strdup(sym_name);
> +	if (!name)
> +		return -errno;
> +
> +	res->syms[res->cnt++] = name;
> +	return 0;
> +}
> +
> +typedef int (*available_kprobe_cb_t)(const char *sym_name, void *ctx);
> +
> +static int
> +libbpf_available_kprobes_parse(available_kprobe_cb_t cb, void *ctx)
> +{
> +	char sym_name[256];
> +	FILE *f;
> +	int ret, err = 0;
> +	const char *available_path = tracefs_available_filter_functions();
> +
> +	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, "%255s%*[^\n]\n", sym_name);
> +		if (ret == EOF && feof(f))
> +			break;
> +		if (ret != 1) {
> +			pr_warn("failed to read available kprobe entry: %d\n",
> +				ret);
> +			err = -EINVAL;
> +			break;
> +		}
> +
> +		err = cb(sym_name, ctx);
> +		if (err)
> +			break;
> +	}
> +
> +	fclose(f);
> +	return err;
> +}
> +
> +static void kprobe_multi_resolve_free(struct kprobe_multi_resolve *res)
> +{
> +	while (res->syms && res->cnt)
> +		free((char *)res->syms[--res->cnt]);
> +
> +	free(res->syms);
> +	free(res->addrs);

I think we also need to zero the res->syms pointer, so the final
kprobe_multi_resolve_free won't try to release it again
perhaps use zfree for both syms and addrs

other than this it looks ok to me:

Acked-by: Jiri Olsa <jolsa@kernel.org>

thanks,
jirka

> +	/* reset cap to zero, when fallback */
> +	res->cap = 0;
> +}
> +
>  struct bpf_link *
>  bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
>  				      const char *pattern,
> @@ -10476,13 +10554,21 @@ 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;
>  		}
> +		syms = res.syms;
>  		addrs = res.addrs;
>  		cnt = res.cnt;
>  	}
> @@ -10511,12 +10597,12 @@ 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
>