libbpf: move feature detection code into its own file

It's quite a lot of well isolated code, so it seems like a good
candidate to move it out of libbpf.c to reduce its size.
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231213190842.3844987-5-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/Build

 libbpf-y := libbpf.o bpf.o nlattr.o btf.o libbpf_errno.o str_error.o \
 	    netlink.o bpf_prog_linfo.o libbpf_probes.o hashmap.o \
 	    btf_dump.o ringbuf.o strset.o linker.o gen_loader.o relo_core.o \
-	    usdt.o zip.o elf.o
+	    usdt.o zip.o elf.o features.o

tools/lib/bpf/elf.c

@@ -11,8 +11,6 @@
 #include "libbpf_internal.h"
 #include "str_error.h"
 
-#define STRERR_BUFSIZE  128
-
 /* A SHT_GNU_versym section holds 16-bit words. This bit is set if
  * the symbol is hidden and can only be seen when referenced using an
  * explicit version number. This is a GNU extension.

tools/lib/bpf/libbpf_internal.h

@@ -360,10 +360,20 @@ enum kern_feature_id {
 	__FEAT_CNT,
 };
 
-struct kern_feature_cache;
+enum kern_feature_result {
+	FEAT_UNKNOWN = 0,
+	FEAT_SUPPORTED = 1,
+	FEAT_MISSING = 2,
+};
+
+struct kern_feature_cache {
+	enum kern_feature_result res[__FEAT_CNT];
+};
+
 bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id);
 bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id);
 
+int probe_kern_syscall_wrapper(void);
 int probe_memcg_account(void);
 int bump_rlimit_memlock(void);
 

tools/lib/bpf/str_error.h

@@ -2,5 +2,8 @@
 #ifndef __LIBBPF_STR_ERROR_H
 #define __LIBBPF_STR_ERROR_H
 
+#define STRERR_BUFSIZE  128
+
 char *libbpf_strerror_r(int err, char *dst, int len);
+
 #endif /* __LIBBPF_STR_ERROR_H */