Subject: Re: [PATCH bpf-next 3/3] bpf: btf: json print map dump with btf info

From: Okash Khawaja <[email-masked]>
Date: Thu, 21 Jun 2018 11:05:21 +0100

On Thu, Jun 21, 2018 at 12:22:52AM +0100, Song Liu wrote:
> 
> 
> > On Jun 20, 2018, at 1:30 PM, Okash Khawaja <[email-masked]> wrote:
> > 
> > This patch modifies `bpftool map dump [-j|-p] id <map-id>` to json-
> > print and pretty-json-print map dump. It calls btf_dumper introduced in
> > previous patch to accomplish this.
> > 
> > The patch only prints debug info when -j or -p flags are supplied. Then
> > too, if the map has associated btf data loaded. Otherwise the usual
> > debug-less output is printed.
> > 
> > Signed-off-by: Okash Khawaja <[email-masked]>
> > Acked-by: Martin KaFai Lau <[email-masked]>
> > 
> > ---
> > tools/bpf/bpftool/map.c |   94 ++++++++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 91 insertions(+), 3 deletions(-)
> > 
> > --- a/tools/bpf/bpftool/map.c
> > +++ b/tools/bpf/bpftool/map.c
> > @@ -43,9 +43,13 @@
> > #include <unistd.h>
> > #include <sys/types.h>
> > #include <sys/stat.h>
> > +#include <linux/err.h>
> > 
> > #include <bpf.h>
> > 
> > +#include "json_writer.h"
> > +#include "btf.h"
> > +#include "btf_dumper.h"
> > #include "main.h"
> > 
> > static const char * const map_type_name[] = {
> > @@ -508,6 +512,83 @@ static int do_show(int argc, char **argv
> > 	return errno == ENOENT ? 0 : -1;
> > }
> > 
> > +
> > +static int do_dump_btf(struct btf *btf, struct bpf_map_info *map_info,
> > +		void *key, void *value)
> > +{
> > +	int ret;
> > +
> > +	jsonw_start_object(json_wtr);
> > +	jsonw_name(json_wtr, "key");
> > +
> > +	ret = btf_dumper_type(btf, json_wtr, map_info->btf_key_type_id, key);
> > +	if (ret)
> > +		goto out;
> > +
> > +	jsonw_end_object(json_wtr);
> > +
> > +	jsonw_start_object(json_wtr);
> > +	jsonw_name(json_wtr, "value");
> > +
> > +	ret = btf_dumper_type(btf, json_wtr, map_info->btf_value_type_id,
> > +			value);
> > +
> > +out:
> > +	/* end of root object */
> > +	jsonw_end_object(json_wtr);
> > +
> > +	return ret;
> > +}
> 
> Please add some tests for do_dump_btf(), including some invalid data type/kind.
Sure. I was wondering if we can follow this up with tests afterwards.
However if they are essential now, I can add them.

> 
> > +
> > +static struct btf *get_btf(struct bpf_map_info *map_info)
> > +{
> > +	int btf_fd = bpf_btf_get_fd_by_id(map_info->btf_id);
> > +	struct bpf_btf_info btf_info = { 0 };
> > +	__u32 len = sizeof(btf_info);
> > +	uint32_t last_size;
> > +	int err;
> > +	struct btf *btf = NULL;
> > +	void *ptr = NULL, *temp_ptr;
> > +
> > +	if (btf_fd < 0)
> > +		return NULL;
> > +
> > +	btf_info.btf_size = 4096;
> 
> Is btf_size always a constant of 4096? 
We start out with 4096 and in the loop below we increase the size if
needed. It seems like a sane valeu to start with. However if there are
better ideas for start value, we can discuss them.

> 
> > +	do {
> > +		last_size = btf_info.btf_size;
> > +		temp_ptr = realloc(ptr, last_size);
> > +		if (!temp_ptr) {
> > +			p_err("unable allocate memory for debug info.");
> > +			goto exit_free;
> > +		}
> > +
> > +		ptr = temp_ptr;
> > +		bzero(ptr, last_size);
> > +		btf_info.btf = ptr_to_u64(ptr);
> > +		err = bpf_obj_get_info_by_fd(btf_fd, &btf_info, &len);
> > +	} while (!err && btf_info.btf_size > last_size && last_size == 4096);
> > +
> > +	if (err || btf_info.btf_size > last_size) {
> > +		p_info("can't get btf info. debug info won't be displayed. error: %s",
> > +				err ? strerror(errno) : "exceeds size retry");
> > +		goto exit_free;
> > +	}
> > +
> > +	btf = btf__new((uint8_t *) btf_info.btf,
> > +			btf_info.btf_size, NULL);
> > +	if (IS_ERR(btf)) {
> > +		printf("error when initialising btf: %s\n",
> > +				strerror(PTR_ERR(btf)));
> > +		btf = NULL;
> > +	}
> > +
> > +exit_free:
> > +	close(btf_fd);
> > +	free(ptr);
> > +
> > +	return btf;
> > +}
> > +
> > static int do_dump(int argc, char **argv)
> > {
> > 	void *key, *value, *prev_key;
> > @@ -516,6 +597,7 @@ static int do_dump(int argc, char **argv
> > 	__u32 len = sizeof(info);
> > 	int err;
> > 	int fd;
> > +	struct btf *btf = NULL;
> > 
> > 	if (argc != 2)
> > 		usage();
> > @@ -538,6 +620,8 @@ static int do_dump(int argc, char **argv
> > 		goto exit_free;
> > 	}
> > 
> > +	btf = get_btf(&info);
> > +
> > 	prev_key = NULL;
> > 	if (json_output)
> > 		jsonw_start_array(json_wtr);
> > @@ -550,9 +634,12 @@ static int do_dump(int argc, char **argv
> > 		}
> > 
> > 		if (!bpf_map_lookup_elem(fd, key, value)) {
> > -			if (json_output)
> > -				print_entry_json(&info, key, value);
> > -			else
> > +			if (json_output) {
> > +				if (btf)
> > +					do_dump_btf(btf, &info, key, value);
> > +				else
> > +					print_entry_json(&info, key, value);
> > +			} else
> > 				print_entry_plain(&info, key, value);
> > 		} else {
> > 			if (json_output) {
> > @@ -584,6 +671,7 @@ exit_free:
> > 	free(key);
> > 	free(value);
> > 	close(fd);
> > +	btf__free(btf);
> > 
> > 	return err;
> > }
> > 
>