Commit 20d59ee5 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Martin KaFai Lau
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libbpf: add bpf_core_cast() macro 



Add bpf_core_cast() macro that wraps bpf_rdonly_cast() kfunc. It's more
ergonomic than kfunc, as it automatically extracts btf_id with
bpf_core_type_id_kernel(), and works with type names. It also casts result
to (T *) pointer. See the definition of the macro, it's self-explanatory.

libbpf declares bpf_rdonly_cast() extern as __weak __ksym and should be
safe to not conflict with other possible declarations in user code.

But we do have a conflict with current BPF selftests that declare their
externs with first argument as `void *obj`, while libbpf opts into more
permissive `const void *obj`. This causes conflict, so we fix up BPF
selftests uses in the same patch.

Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240130212023.183765-2-andrii@kernel.org


Signed-off-by: default avatarMartin KaFai Lau <martin.lau@kernel.org>
parent 4d8ebe13

tools/lib/bpf/bpf_core_read.h

+13 −0
Original line number Diff line number Diff line
@@ -2,6 +2,8 @@ 
#ifndef __BPF_CORE_READ_H__

#define __BPF_CORE_READ_H__



#include <bpf/bpf_helpers.h>



/*

 * enum bpf_field_info_kind is passed as a second argument into

 * __builtin_preserve_field_info() built-in to get a specific aspect of
@@ -292,6 +294,17 @@ enum bpf_enum_value_kind { 
#define bpf_core_read_user_str(dst, sz, src)				    \

	bpf_probe_read_user_str(dst, sz, (const void *)__builtin_preserve_access_index(src))



extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;



/*

 * Cast provided pointer *ptr* into a pointer to a specified *type* in such

 * a way that BPF verifier will become aware of associated kernel-side BTF

 * type. This allows to access members of kernel types directly without the

 * need to use BPF_CORE_READ() macros.

 */

#define bpf_core_cast(ptr, type)					    \

	((typeof(type) *)bpf_rdonly_cast((ptr), bpf_core_type_id_kernel(type)))



#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

tools/testing/selftests/bpf/bpf_kfuncs.h

+1 −1
Original line number Diff line number Diff line
@@ -63,7 +63,7 @@ extern int bpf_sk_assign_tcp_reqsk(struct __sk_buff *skb, struct sock *sk, 


void *bpf_cast_to_kern_ctx(void *) __ksym;



void *bpf_rdonly_cast(void *obj, __u32 btf_id) __ksym;

extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;



extern int bpf_get_file_xattr(struct file *file, const char *name,

			      struct bpf_dynptr *value_ptr) __ksym;

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

+0 −2
Original line number Diff line number Diff line
@@ -12,8 +12,6 @@ int cookie_found = 0; 
__u64 cookie = 0;

__u32 omem = 0;



void *bpf_rdonly_cast(void *, __u32) __ksym;



struct {

	__uint(type, BPF_MAP_TYPE_SK_STORAGE);

	__uint(map_flags, BPF_F_NO_PREALLOC);

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

+1 −3
Original line number Diff line number Diff line
@@ -4,6 +4,7 @@ 
#include <bpf/bpf_helpers.h>

#include <bpf/bpf_tracing.h>

#include <bpf/bpf_core_read.h>

#include "bpf_kfuncs.h"



struct {

	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
@@ -19,9 +20,6 @@ char name[IFNAMSIZ]; 
unsigned int inum;

unsigned int meta_len, frag0_len, kskb_len, kskb2_len;



void *bpf_cast_to_kern_ctx(void *) __ksym;

void *bpf_rdonly_cast(void *, __u32) __ksym;



SEC("?xdp")

int md_xdp(struct xdp_md *ctx)

{