Re: [PATCH bpf-next v3 2/3] bpf: btf: add btf print functionality

Jakub Kicinski
2018-07-10T03:56:31Z

On Sun, 8 Jul 2018 13:30:04 -0700, Okash Khawaja wrote:
> This consumes functionality exported in the previous patch. It does the
> main job of printing with BTF data. This is used in the following patch
> to provide a more readable output of a map's dump. It relies on
> json_writer to do json printing. Below is sample output where map keys
> are ints and values are of type struct A:
>
> typedef int int_type;
> enum E {
> E0,
> E1,
> };
>
> struct B {
> int x;
> int y;
> };
>
> struct A {
> int m;
> unsigned long long n;
> char o;
> int p[8];
> int q[4][8];
> enum E r;
> void *s;
> struct B t;
> const int u;
> int_type v;
> unsigned int w1: 3;
> unsigned int w2: 3;
> };
>
> $ sudo bpftool map dump id 14
> [{
> "key": 0,
> "value": {
> "m": 1,
> "n": 2,
> "o": "c",
> "p": [15,16,17,18,15,16,17,18
> ],
> "q": [[25,26,27,28,25,26,27,28
> ],[35,36,37,38,35,36,37,38
> ],[45,46,47,48,45,46,47,48
> ],[55,56,57,58,55,56,57,58
> ]
> ],
> "r": 1,
> "s": 0x7ffd80531cf8,
> "t": {
> "x": 5,
> "y": 10
> },
> "u": 100,
> "v": 20,
> "w1": 0x7,
> "w2": 0x3
> }
> }
> ]
>
> This patch uses json's {} and [] to imply struct/union and array. More
> explicit information can be added later. For example, a command line
> option can be introduced to print whether a key or value is struct
> or union, name of a struct etc. This will however come at the expense
> of duplicating info when, for example, printing an array of structs.
> enums are printed as ints without their names.
>
> Signed-off-by: Okash Khawaja <osk@fb.com>
> Acked-by: Martin KaFai Lau <kafai@fb.com>
>
> ---
> tools/bpf/bpftool/btf_dumper.c | 253 +++++++++++++++++++++++++++++++++++++++++
> tools/bpf/bpftool/main.h | 15 ++
> 2 files changed, 268 insertions(+)
>
> --- /dev/null
> +++ b/tools/bpf/bpftool/btf_dumper.c
> @@ -0,0 +1,253 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2018 Facebook */
> +
> +#include <linux/btf.h>
> +#include <linux/err.h>
> +#include <stdio.h> /* for (FILE *) used by json_writer */
> +#include <linux/bitops.h>
> +#include <string.h>
> +#include <ctype.h>

fwiw: the preferred ordering would have been:

#include <ctype.h>
#include <stdio.h> /* for (FILE *) used by json_writer */
#include <string.h>
#include <linux/bitops.h>
#include <linux/btf.h>
#include <linux/err.h>

> +#include "btf.h"
> +#include "json_writer.h"
> +#include "main.h"
> +
> +#define BITS_PER_BYTE_MASK (BITS_PER_BYTE - 1)
> +#define BITS_PER_BYTE_MASKED(bits) ((bits) & BITS_PER_BYTE_MASK)
> +#define BITS_ROUNDDOWN_BYTES(bits) ((bits) >> 3)
> +#define BITS_ROUNDUP_BYTES(bits) \
> + (BITS_ROUNDDOWN_BYTES(bits) + !!BITS_PER_BYTE_MASKED(bits))
> +const int one = 1;
> +#define is_big_endian() ((*(char *)&one) == 0)

Could we try to do this at compilation time? Without the variable? :(

#include <asm/byteorder.h>

#if defined(__BYTE_ORDER) ? __BYTE_ORDER == __BIG_ENDIAN : defined(__BIG_ENDIAN)
return true;
#else
return false;
#endif

We could also just include endian.h, but since it's a non-standard
extension perhaps using kernel header is a safer bet.

> +static int btf_dumper_do_type(const struct btf_dumper *d, __u32 type_id,
> + __u8 bit_offset, const void *data);
> +
> +static void btf_dumper_ptr(const void *data, json_writer_t *jw,
> + bool is_plain_text)
> +{
> + if (is_plain_text)
> + jsonw_printf(jw, "%p", *((unsigned long *)data));
> + else
> + jsonw_printf(jw, "%u", *((unsigned long *)data));

nit: I think you missed these parenthesis

> +}
> +

> +static void btf_dumper_int_bits(__u32 int_type, __u8 bit_offset,
> + const void *data, json_writer_t *jw,
> + bool is_plain_text)
> +{
> + int left_shift_bits, right_shift_bits;
> + int nr_bits = BTF_INT_BITS(int_type);
> + int total_bits_offset;
> + int bytes_to_copy;
> + int bits_to_copy;
> + __u64 print_num;
> +
> + total_bits_offset = bit_offset + BTF_INT_OFFSET(int_type);
> + data += BITS_ROUNDDOWN_BYTES(total_bits_offset);
> + bit_offset = BITS_PER_BYTE_MASKED(total_bits_offset);
> + bits_to_copy = bit_offset + nr_bits;
> + bytes_to_copy = BITS_ROUNDUP_BYTES(bits_to_copy);
> +
> + print_num = 0;
> + memcpy(&print_num, data, bytes_to_copy);
> + if (is_big_endian()) {
> + left_shift_bits = bit_offset;
> + right_shift_bits = 64 - nr_bits;
> + } else {
> + left_shift_bits = 64 - bits_to_copy;
> + right_shift_bits = 64 - nr_bits;
> + }

Or you can just put the #if here, since it's the only use.

> + print_num <<= left_shift_bits;
> + print_num >>= right_shift_bits;
> + if (is_plain_text)
> + jsonw_printf(jw, "0x%llx", print_num);
> + else
> + jsonw_printf(jw, "%llu", print_num);
> +}
> +
> +static int btf_dumper_int(const struct btf_type *t, __u8 bit_offset,
> + const void *data, json_writer_t *jw,
> + bool is_plain_text)
> +{
> + __u32 *int_type;
> + __u32 nr_bits;
> +
> + int_type = (__u32 *)(t + 1);
> + nr_bits = BTF_INT_BITS(*int_type);
> + /* if this is bit field */
> + if (bit_offset || BTF_INT_OFFSET(*int_type) ||
> + BITS_PER_BYTE_MASKED(nr_bits)) {
> + btf_dumper_int_bits(*int_type, bit_offset, data, jw,
> + is_plain_text);
> + return 0;
> + }
> +
> + switch (BTF_INT_ENCODING(*int_type)) {
> + case 0:
> + if (BTF_INT_BITS(*int_type) == 64)
> + jsonw_printf(jw, "%lu", *((__u64 *)data));

nit: more parenthesis here

> + else if (BTF_INT_BITS(*int_type) == 32)
> + jsonw_printf(jw, "%u", *((__u32 *)data));
> + else if (BTF_INT_BITS(*int_type) == 16)
> + jsonw_printf(jw, "%hu", *((__u16 *)data));
> + else if (BTF_INT_BITS(*int_type) == 8)
> + jsonw_printf(jw, "%hhu", *((__u8 *)data));
> + else
> + btf_dumper_int_bits(*int_type, bit_offset, data, jw,
> + is_plain_text);
> + break;
> + case BTF_INT_SIGNED:
> + if (BTF_INT_BITS(*int_type) == 64)
> + jsonw_printf(jw, "%ld", *((long long *)data));
> + else if (BTF_INT_BITS(*int_type) == 32)
> + jsonw_printf(jw, "%d", *((int *)data));
> + else if (BTF_INT_BITS(*int_type) == 16)
> + jsonw_printf(jw, "%hd", *((short *)data));
> + else if (BTF_INT_BITS(*int_type) == 8)
> + jsonw_printf(jw, "%hhd", *((char *)data));
> + else
> + btf_dumper_int_bits(*int_type, bit_offset, data, jw,
> + is_plain_text);
> + break;
> + case BTF_INT_CHAR:
> + if (*((char *)data) == '\0')

nit: here too, etc..

> + jsonw_null(jw);

I don't think the null is good. I thought I mentioned that? Look for
example at Python:

>>> import json
>>> thing = json.loads('{"a": [97, 98, 99, 100]}')
>>> bytearray(thing["str"]).decode('utf-8')
'abcd'
>>> "".join(map(chr, thing["str"]))
'abcd'
>>> thing = json.loads('{"str": [97, 98, 99, 100, null]}')
>>> bytearray(thing["str"]).decode('utf-8')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: an integer is required
>>> "".join(map(chr, thing["str"]))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: an integer is required (got type NoneType)

If you start putting nulls into the array the conversion to a string
will become more difficult, won't it? Do you have a use case where
this helps? Maybe my Python-foo is not strong enough?

> + else if (isprint(*((char *)data)))
> + jsonw_printf(jw, "\"%c\"", *((char *)data));
> + else
> + if (is_plain_text)
> + jsonw_printf(jw, "0x%hhx", *((char *)data));
> + else
> + jsonw_printf(jw, "\"\\u00%02hhx\"",
> + *((char *)data));
> + break;
> + case BTF_INT_BOOL:
> + jsonw_bool(jw, *((int *)data));
> + break;
> + default:
> + /* shouldn't happen */
> + return -EINVAL;
> + }
> +
> + return 0;
> +}
> +
> +static int btf_dumper_struct(const struct btf_dumper *d, __u32 type_id,
> + const void *data)
> +{
> + const struct btf_type *t;
> + struct btf_member *m;
> + int ret = 0;
> + int i, vlen;
> +
> + t = btf__type_by_id(d->btf, type_id);
> + if (!t)
> + return -EINVAL;
> +
> + vlen = BTF_INFO_VLEN(t->info);
> + jsonw_start_object(d->jw);
> + m = (struct btf_member *)(t + 1);
> +
> + for (i = 0; i < vlen; i++) {
> + const void *data_off = data +
> + BITS_ROUNDDOWN_BYTES(m[i].offset);

nit: empty line between variable declaration and code, perhaps also
don't init inline since it doesn't fit that way?

> + jsonw_name(d->jw, btf__name_by_offset(d->btf, m[i].name_off));
> + ret = btf_dumper_do_type(d, m[i].type,
> + BITS_PER_BYTE_MASKED(m[i].offset),
> + data_off);
> + if (ret)
> + break;
> + }
> +
> + jsonw_end_object(d->jw);
> +
> + return ret;
> +}

Thanks for all the changes you've made so far!