Commit 7db3822a authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Daniel Borkmann
Browse files

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: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Acked-by: default avatarSong Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191008175942.1769476-7-andriin@fb.com
parent e01a75c1

tools/lib/bpf/Makefile

+2 −1
Original line number Diff line number Diff line
@@ -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)) \

tools/lib/bpf/bpf_core_read.h

0 → 100644
+167 −0
Original line number Diff line number Diff line 
/* 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

tools/lib/bpf/bpf_helpers.h

+9 −23
Original line number Diff line number Diff line
@@ -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

tools/testing/selftests/bpf/progs/test_core_reloc_arrays.c

+1 −0
Original line number Diff line number Diff line
@@ -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";

tools/testing/selftests/bpf/progs/test_core_reloc_flavors.c

+1 −0
Original line number Diff line number Diff line
@@ -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";

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