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

From: Yonghong Song <yhs@fb.com>
To: Jiri Olsa <jolsa@redhat.com>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Yinjun Zhang <yinjun.zhang@corigine.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>,
	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: Mon, 7 Feb 2022 12:34:47 -0800	[thread overview]
Message-ID: <ab1e74e8-ce5a-44b8-47cb-3f326973bb59@fb.com> (raw)
In-Reply-To: <YgFkVXggmihEpO/o@krava>



On 2/7/22 10:26 AM, 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/

For the patch in the above link:

diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
index c66a3c979b7a..2ccf85042e75 100644
--- a/tools/bpf/bpftool/map.c
+++ b/tools/bpf/bpftool/map.c
@@ -862,6 +862,7 @@ map_dump(int fd, struct bpf_map_info *info, 
json_writer_t *wtr,
  	prev_key = NULL;

  	if (wtr) {
+		errno = 0;
  		btf = get_map_kv_btf(info);
  		err = libbpf_get_error(btf);
  		if (err) {
-- 

Do we know who sets non-zero errno in the above?
Maybe we can fix the issue in that place?

>>
>> 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
> 
> 
> ---
>   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);
