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: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240130212023.183765-2-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

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

tools/testing/selftests/bpf/bpf_kfuncs.h

@@ -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

@@ -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

@@ -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)
 {