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libbpf: Add BPF_CORE_READ/BPF_CORE_READ_INTO helpers 

Add few macros simplifying BCC-like multi-level probe reads, while also
emitting CO-RE relocations for each read.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191008175942.1769476-7-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2019-10-08 10:59:41 -07:00 committed by Daniel Borkmann
parent e01a75c159
commit 7db3822ab9
12 changed files with 187 additions and 24 deletions
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3
tools/lib/bpf/Makefile
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@ -250,7 +250,8 @@ install_headers: bpf_helper_defs.h
$(call do_install,bpf_helpers.h,$(prefix)/include/bpf,644); \
$(call do_install,bpf_helper_defs.h,$(prefix)/include/bpf,644); \
$(call do_install,bpf_tracing.h,$(prefix)/include/bpf,644); \
$(call do_install,bpf_endian.h,$(prefix)/include/bpf,644);
$(call do_install,bpf_endian.h,$(prefix)/include/bpf,644); \
$(call do_install,bpf_core_read.h,$(prefix)/include/bpf,644);
install_pkgconfig: $(PC_FILE)
$(call QUIET_INSTALL, $(PC_FILE)) \

167
tools/lib/bpf/bpf_core_read.h Normal file
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@ -0,0 +1,167 @@
/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
#ifndef __BPF_CORE_READ_H__
#define __BPF_CORE_READ_H__
/*
* bpf_core_read() abstracts away bpf_probe_read() call and captures offset
* relocation for source address using __builtin_preserve_access_index()
* built-in, provided by Clang.
*
* __builtin_preserve_access_index() takes as an argument an expression of
* taking an address of a field within struct/union. It makes compiler emit
* a relocation, which records BTF type ID describing root struct/union and an
* accessor string which describes exact embedded field that was used to take
* an address. See detailed description of this relocation format and
* semantics in comments to struct bpf_offset_reloc in libbpf_internal.h.
*
* This relocation allows libbpf to adjust BPF instruction to use correct
* actual field offset, based on target kernel BTF type that matches original
* (local) BTF, used to record relocation.
*/
#define bpf_core_read(dst, sz, src) \
bpf_probe_read(dst, sz, \
(const void *)__builtin_preserve_access_index(src))
/*
* bpf_core_read_str() is a thin wrapper around bpf_probe_read_str()
* additionally emitting BPF CO-RE field relocation for specified source
* argument.
*/
#define bpf_core_read_str(dst, sz, src) \
bpf_probe_read_str(dst, sz, \
(const void *)__builtin_preserve_access_index(src))
#define ___concat(a, b) a ## b
#define ___apply(fn, n) ___concat(fn, n)
#define ___nth(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, __11, N, ...) N
/*
* return number of provided arguments; used for switch-based variadic macro
* definitions (see ___last, ___arrow, etc below)
*/
#define ___narg(...) ___nth(_, ##__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
/*
* return 0 if no arguments are passed, N - otherwise; used for
* recursively-defined macros to specify termination (0) case, and generic
* (N) case (e.g., ___read_ptrs, ___core_read)
*/
#define ___empty(...) ___nth(_, ##__VA_ARGS__, N, N, N, N, N, N, N, N, N, N, 0)
#define ___last1(x) x
#define ___last2(a, x) x
#define ___last3(a, b, x) x
#define ___last4(a, b, c, x) x
#define ___last5(a, b, c, d, x) x
#define ___last6(a, b, c, d, e, x) x
#define ___last7(a, b, c, d, e, f, x) x
#define ___last8(a, b, c, d, e, f, g, x) x
#define ___last9(a, b, c, d, e, f, g, h, x) x
#define ___last10(a, b, c, d, e, f, g, h, i, x) x
#define ___last(...) ___apply(___last, ___narg(__VA_ARGS__))(__VA_ARGS__)
#define ___nolast2(a, _) a
#define ___nolast3(a, b, _) a, b
#define ___nolast4(a, b, c, _) a, b, c
#define ___nolast5(a, b, c, d, _) a, b, c, d
#define ___nolast6(a, b, c, d, e, _) a, b, c, d, e
#define ___nolast7(a, b, c, d, e, f, _) a, b, c, d, e, f
#define ___nolast8(a, b, c, d, e, f, g, _) a, b, c, d, e, f, g
#define ___nolast9(a, b, c, d, e, f, g, h, _) a, b, c, d, e, f, g, h
#define ___nolast10(a, b, c, d, e, f, g, h, i, _) a, b, c, d, e, f, g, h, i
#define ___nolast(...) ___apply(___nolast, ___narg(__VA_ARGS__))(__VA_ARGS__)
#define ___arrow1(a) a
#define ___arrow2(a, b) a->b
#define ___arrow3(a, b, c) a->b->c
#define ___arrow4(a, b, c, d) a->b->c->d
#define ___arrow5(a, b, c, d, e) a->b->c->d->e
#define ___arrow6(a, b, c, d, e, f) a->b->c->d->e->f
#define ___arrow7(a, b, c, d, e, f, g) a->b->c->d->e->f->g
#define ___arrow8(a, b, c, d, e, f, g, h) a->b->c->d->e->f->g->h
#define ___arrow9(a, b, c, d, e, f, g, h, i) a->b->c->d->e->f->g->h->i
#define ___arrow10(a, b, c, d, e, f, g, h, i, j) a->b->c->d->e->f->g->h->i->j
#define ___arrow(...) ___apply(___arrow, ___narg(__VA_ARGS__))(__VA_ARGS__)
#define ___type(...) typeof(___arrow(__VA_ARGS__))
#define ___read(read_fn, dst, src_type, src, accessor) \
read_fn((void *)(dst), sizeof(*(dst)), &((src_type)(src))->accessor)
/* "recursively" read a sequence of inner pointers using local __t var */
#define ___rd_last(...) \
___read(bpf_core_read, &__t, \
___type(___nolast(__VA_ARGS__)), __t, ___last(__VA_ARGS__));
#define ___rd_p0(src) const void *__t = src;
#define ___rd_p1(...) ___rd_p0(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p2(...) ___rd_p1(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p3(...) ___rd_p2(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p4(...) ___rd_p3(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p5(...) ___rd_p4(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p6(...) ___rd_p5(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p7(...) ___rd_p6(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p8(...) ___rd_p7(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p9(...) ___rd_p8(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___read_ptrs(src, ...) \
___apply(___rd_p, ___narg(__VA_ARGS__))(src, __VA_ARGS__)
#define ___core_read0(fn, dst, src, a) \
___read(fn, dst, ___type(src), src, a);
#define ___core_readN(fn, dst, src, ...) \
___read_ptrs(src, ___nolast(__VA_ARGS__)) \
___read(fn, dst, ___type(src, ___nolast(__VA_ARGS__)), __t, \
___last(__VA_ARGS__));
#define ___core_read(fn, dst, src, a, ...) \
___apply(___core_read, ___empty(__VA_ARGS__))(fn, dst, \
src, a, ##__VA_ARGS__)
/*
* BPF_CORE_READ_INTO() is a more performance-conscious variant of
* BPF_CORE_READ(), in which final field is read into user-provided storage.
* See BPF_CORE_READ() below for more details on general usage.
*/
#define BPF_CORE_READ_INTO(dst, src, a, ...) \
({ \
___core_read(bpf_core_read, dst, src, a, ##__VA_ARGS__) \
})
/*
* BPF_CORE_READ_STR_INTO() does same "pointer chasing" as
* BPF_CORE_READ() for intermediate pointers, but then executes (and returns
* corresponding error code) bpf_core_read_str() for final string read.
*/
#define BPF_CORE_READ_STR_INTO(dst, src, a, ...) \
({ \
___core_read(bpf_core_read_str, dst, src, a, ##__VA_ARGS__) \
})
/*
* BPF_CORE_READ() is used to simplify BPF CO-RE relocatable read, especially
* when there are few pointer chasing steps.
* E.g., what in non-BPF world (or in BPF w/ BCC) would be something like:
* int x = s->a.b.c->d.e->f->g;
* can be succinctly achieved using BPF_CORE_READ as:
* int x = BPF_CORE_READ(s, a.b.c, d.e, f, g);
*
* BPF_CORE_READ will decompose above statement into 4 bpf_core_read (BPF
* CO-RE relocatable bpf_probe_read() wrapper) calls, logically equivalent to:
* 1. const void *__t = s->a.b.c;
* 2. __t = __t->d.e;
* 3. __t = __t->f;
* 4. return __t->g;
*
* Equivalence is logical, because there is a heavy type casting/preservation
* involved, as well as all the reads are happening through bpf_probe_read()
* calls using __builtin_preserve_access_index() to emit CO-RE relocations.
*
* N.B. Only up to 9 "field accessors" are supported, which should be more
* than enough for any practical purpose.
*/
#define BPF_CORE_READ(src, a, ...) \
({ \
___type(src, a, ##__VA_ARGS__) __r; \
BPF_CORE_READ_INTO(&__r, src, a, ##__VA_ARGS__); \
__r; \
})
#endif

32
tools/lib/bpf/bpf_helpers.h
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@ -7,7 +7,7 @@
#define __uint(name, val) int (*name)[val]
#define __type(name, val) typeof(val) *name
/* helper macro to print out debug messages */
/* Helper macro to print out debug messages */
#define bpf_printk(fmt, ...) \
({ \
char ____fmt[] = fmt; \
@ -15,13 +15,19 @@
##__VA_ARGS__); \
})
/* helper macro to place programs, maps, license in
/*
* Helper macro to place programs, maps, license in
* different sections in elf_bpf file. Section names
* are interpreted by elf_bpf loader
*/
#define SEC(NAME) __attribute__((section(NAME), used))
/* a helper structure used by eBPF C program
#ifndef __always_inline
#define __always_inline __attribute__((always_inline))
#endif
/*
* Helper structure used by eBPF C program
* to describe BPF map attributes to libbpf loader
*/
struct bpf_map_def {
@ -32,24 +38,4 @@ struct bpf_map_def {
unsigned int map_flags;
};
/*
* bpf_core_read() abstracts away bpf_probe_read() call and captures offset
* relocation for source address using __builtin_preserve_access_index()
* built-in, provided by Clang.
*
* __builtin_preserve_access_index() takes as an argument an expression of
* taking an address of a field within struct/union. It makes compiler emit
* a relocation, which records BTF type ID describing root struct/union and an
* accessor string which describes exact embedded field that was used to take
* an address. See detailed description of this relocation format and
* semantics in comments to struct bpf_offset_reloc in libbpf_internal.h.
*
* This relocation allows libbpf to adjust BPF instruction to use correct
* actual field offset, based on target kernel BTF type that matches original
* (local) BTF, used to record relocation.
*/
#define bpf_core_read(dst, sz, src) \
bpf_probe_read(dst, sz, \
(const void *)__builtin_preserve_access_index(src))
#endif

1
tools/testing/selftests/bpf/progs/test_core_reloc_arrays.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_flavors.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_ints.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_kernel.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_misc.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_mods.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_nesting.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_primitives.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";

1
tools/testing/selftests/bpf/progs/test_core_reloc_ptr_as_arr.c
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@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <stdint.h>
#include "bpf_helpers.h"
#include "bpf_core_read.h"
char _license[] SEC("license") = "GPL";