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.
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
tools/lib/bpf/bpf_core_read.h | 13 +++++++++++++
tools/testing/selftests/bpf/bpf_kfuncs.h | 2 +-
.../selftests/bpf/progs/sk_storage_omem_uncharge.c | 2 --
tools/testing/selftests/bpf/progs/type_cast.c | 4 +---
4 files changed, 15 insertions(+), 6 deletions(-)
diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index 7325a12692a3..4fdb7a7320d6 100644
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -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
diff --git a/tools/testing/selftests/bpf/bpf_kfuncs.h b/tools/testing/selftests/bpf/bpf_kfuncs.h
index b4e78c1eb37b..24dafb289190 100644
--- a/tools/testing/selftests/bpf/bpf_kfuncs.h
+++ b/tools/testing/selftests/bpf/bpf_kfuncs.h
@@ -53,7 +53,7 @@ extern int bpf_sock_addr_set_sun_path(struct bpf_sock_addr_kern *sa_kern,
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;
diff --git a/tools/testing/selftests/bpf/progs/sk_storage_omem_uncharge.c b/tools/testing/selftests/bpf/progs/sk_storage_omem_uncharge.c
index 3e745793b27a..934c4e5ada5b 100644
--- a/tools/testing/selftests/bpf/progs/sk_storage_omem_uncharge.c
+++ b/tools/testing/selftests/bpf/progs/sk_storage_omem_uncharge.c
@@ -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);
diff --git a/tools/testing/selftests/bpf/progs/type_cast.c b/tools/testing/selftests/bpf/progs/type_cast.c
index a9629ac230fd..98ab35893f51 100644
--- a/tools/testing/selftests/bpf/progs/type_cast.c
+++ b/tools/testing/selftests/bpf/progs/type_cast.c
@@ -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)
{
--
2.34.1
