Re: [PATCH bpf] bpftool: fix the error when lookup in no-btf maps

From: Yinjun Zhang <yinjun.zhang@corigine.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, Martin Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	john fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	niklas.soderlund@corigine.com,
	Simon Horman <simon.horman@corigine.com>
Subject: Re: [PATCH bpf] bpftool: fix the error when lookup in no-btf maps
Date: Tue, 8 Feb 2022 12:53:43 +0800	[thread overview]
Message-ID: <20220208045343.GA16157@nj-rack01-04.nji.corigine.com> (raw)
In-Reply-To: <YgFkVXggmihEpO/o@krava>

On Mon, Feb 07, 2022 at 07:26:29PM +0100, Jiri Olsa wrote:
> On Mon, Feb 07, 2022 at 06:57:20PM +0100, Jiri Olsa wrote:
> > On Mon, Feb 07, 2022 at 09:42:25AM -0800, Andrii Nakryiko wrote:
> > > On Mon, Feb 7, 2022 at 8:00 AM Yinjun Zhang <yinjun.zhang@corigine.com> wrote:
> > > >
> > > > When reworking btf__get_from_id() in commit a19f93cfafdf the error
> > > > handling when calling bpf_btf_get_fd_by_id() changed. Before the rework
> > > > if bpf_btf_get_fd_by_id() failed the error would not be propagated to
> > > > callers of btf__get_from_id(), after the rework it is. This lead to a
> > > > change in behavior in print_key_value() that now prints an error when
> > > > trying to lookup keys in maps with no btf available.
> > > >
> > > > Fix this by following the way used in dumping maps to allow to look up
> > > > keys in no-btf maps, by which it decides whether and where to get the
> > > > btf info according to the btf value type.
> > > >
> > > > Fixes: a19f93cfafdf ("libbpf: Add internal helper to load BTF data by FD")
> > > > Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
> > > > Reviewed-by: Niklas Söderlund <niklas.soderlund@corigine.com>
> > > > Signed-off-by: Simon Horman <simon.horman@corigine.com>
> > > > ---
> > > >  tools/bpf/bpftool/map.c | 6 ++----
> > > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> > > > index cc530a229812..4fc772d66e3a 100644
> > > > --- a/tools/bpf/bpftool/map.c
> > > > +++ b/tools/bpf/bpftool/map.c
> > > > @@ -1054,11 +1054,9 @@ static void print_key_value(struct bpf_map_info *info, void *key,
> > > >         json_writer_t *btf_wtr;
> > > >         struct btf *btf;
> > > >
> > > > -       btf = btf__load_from_kernel_by_id(info->btf_id);
> > > > -       if (libbpf_get_error(btf)) {
> > > > -               p_err("failed to get btf");
> > > > +       btf = get_map_kv_btf(info);
> > > > +       if (libbpf_get_error(btf))
> > > 
> > > See discussion in [0], it seems relevant.
> > > 
> > >   [0] https://lore.kernel.org/bpf/20220204225823.339548-3-jolsa@kernel.org/
> > 
> > I checked and this patch does not fix the problem for me,
> > but looks like similar issue, do you have test case for this?
> > 
> > mine is to dump any no-btf map with -p option
> 
> anyway I think your change should go in separately,
> I can send change below (v2 for [0] above) on top of yours
> 
> thanks,
> jirka
> 

Yeah, I agree they are separate issues, and my test case is also
simple, just lookup a key in a no-btf map, example:
# ./bpftool map create /sys/fs/bpf/xx type array key 4 value 4 entries 2 name def
# ./bpftool map lookup name def key 0 0 0 0
Error: failed to get btf

> 
> ---
>  tools/bpf/bpftool/map.c | 31 +++++++++++++------------------
>  1 file changed, 13 insertions(+), 18 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index c66a3c979b7a..8562add7417d 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c
> @@ -805,29 +805,28 @@ static int maps_have_btf(int *fds, int nb_fds)
>  
>  static struct btf *btf_vmlinux;
>  
> -static struct btf *get_map_kv_btf(const struct bpf_map_info *info)
> +static int get_map_kv_btf(const struct bpf_map_info *info, struct btf **btf)
>  {
> -	struct btf *btf = NULL;
> +	int err = 0;
>  
>  	if (info->btf_vmlinux_value_type_id) {
>  		if (!btf_vmlinux) {
>  			btf_vmlinux = libbpf_find_kernel_btf();
> -			if (libbpf_get_error(btf_vmlinux))
> +			err = libbpf_get_error(btf_vmlinux);
> +			if (err) {
>  				p_err("failed to get kernel btf");
> +				return err;
> +			}
>  		}
> -		return btf_vmlinux;
> +		*btf = btf_vmlinux;
>  	} else if (info->btf_value_type_id) {
> -		int err;
> -
> -		btf = btf__load_from_kernel_by_id(info->btf_id);
> -		err = libbpf_get_error(btf);
> -		if (err) {
> +		*btf = btf__load_from_kernel_by_id(info->btf_id);
> +		err = libbpf_get_error(*btf);
> +		if (err)
>  			p_err("failed to get btf");
> -			btf = ERR_PTR(err);
> -		}
>  	}
>  
> -	return btf;
> +	return err;
>  }
>  
>  static void free_map_kv_btf(struct btf *btf)
> @@ -862,8 +861,7 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
>  	prev_key = NULL;
>  
>  	if (wtr) {
> -		btf = get_map_kv_btf(info);
> -		err = libbpf_get_error(btf);
> +		err = get_map_kv_btf(info, &btf);
>  		if (err) {
>  			goto exit_free;
>  		}
> @@ -1054,11 +1052,8 @@ static void print_key_value(struct bpf_map_info *info, void *key,
>  	json_writer_t *btf_wtr;
>  	struct btf *btf;
>  
> -	btf = btf__load_from_kernel_by_id(info->btf_id);
> -	if (libbpf_get_error(btf)) {
> -		p_err("failed to get btf");
> +	if (get_map_kv_btf(info, &btf))
>  		return;
> -	}
>  
>  	if (json_output) {
>  		print_entry_json(info, key, value, btf);
> -- 
> 2.34.1
>