libbpf: stop enforcing kern_version, populate it for users

Kernel version enforcement for kprobes/kretprobes was removed from
5.0 kernel in 6c4fc209 ("bpf: remove useless version check for prog load").
Since then, BPF programs were specifying SEC("version") just to please
libbpf. We should stop enforcing this in libbpf, if even kernel doesn't
care. Furthermore, libbpf now will pre-populate current kernel version
of the host system, in case we are still running on old kernel.

This patch also removes __bpf_object__open_xattr from libbpf.h, as
nothing in libbpf is relying on having it in that header. That function
was never exported as LIBBPF_API and even name suggests its internal
version. So this should be safe to remove, as it doesn't break ABI.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -33,6 +33,7 @@
 #include <linux/limits.h>
 #include <linux/perf_event.h>
 #include <linux/ring_buffer.h>
+#include <linux/version.h>
 #include <sys/epoll.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
@@ -255,7 +256,7 @@ struct bpf_object {
 	 */
 	struct {
 		int fd;
-		void *obj_buf;
+		const void *obj_buf;
 		size_t obj_buf_sz;
 		Elf *elf;
 		GElf_Ehdr ehdr;
@@ -491,8 +492,19 @@ bpf_object__init_prog_names(struct bpf_object *obj)
 	return 0;
 }
 
+static __u32 get_kernel_version(void)
+{
+	__u32 major, minor, patch;
+	struct utsname info;
+
+	uname(&info);
+	if (sscanf(info.release, "%u.%u.%u", &major, &minor, &patch) != 3)
+		return 0;
+	return KERNEL_VERSION(major, minor, patch);
+}
+
 static struct bpf_object *bpf_object__new(const char *path,
-					  void *obj_buf,
+					  const void *obj_buf,
 					  size_t obj_buf_sz)
 {
 	struct bpf_object *obj;
@@ -526,6 +538,7 @@ static struct bpf_object *bpf_object__new(const char *path,
 	obj->efile.rodata_shndx = -1;
 	obj->efile.bss_shndx = -1;
 
+	obj->kern_version = get_kernel_version();
 	obj->loaded = false;
 
 	INIT_LIST_HEAD(&obj->list);
@@ -569,7 +582,7 @@ static int bpf_object__elf_init(struct bpf_object *obj)
 		 * obj_buf should have been validated by
 		 * bpf_object__open_buffer().
 		 */
-		obj->efile.elf = elf_memory(obj->efile.obj_buf,
+		obj->efile.elf = elf_memory((char *)obj->efile.obj_buf,
 					    obj->efile.obj_buf_sz);
 	} else {
 		obj->efile.fd = open(obj->path, O_RDONLY);
@@ -636,21 +649,6 @@ bpf_object__init_license(struct bpf_object *obj, void *data, size_t size)
 	return 0;
 }
 
-static int
-bpf_object__init_kversion(struct bpf_object *obj, void *data, size_t size)
-{
-	__u32 kver;
-
-	if (size != sizeof(kver)) {
-		pr_warning("invalid kver section in %s\n", obj->path);
-		return -LIBBPF_ERRNO__FORMAT;
-	}
-	memcpy(&kver, data, sizeof(kver));
-	obj->kern_version = kver;
-	pr_debug("kernel version of %s is %x\n", obj->path, obj->kern_version);
-	return 0;
-}
-
 static int compare_bpf_map(const void *_a, const void *_b)
 {
 	const struct bpf_map *a = _a;
@@ -1568,11 +1566,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
 			if (err)
 				return err;
 		} else if (strcmp(name, "version") == 0) {
-			err = bpf_object__init_kversion(obj,
-							data->d_buf,
-							data->d_size);
-			if (err)
-				return err;
+			/* skip, we don't need it anymore */
 		} else if (strcmp(name, "maps") == 0) {
 			obj->efile.maps_shndx = idx;
 		} else if (strcmp(name, MAPS_ELF_SEC) == 0) {
@@ -3551,54 +3545,9 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
 	return 0;
 }
 
-static bool bpf_prog_type__needs_kver(enum bpf_prog_type type)
-{
-	switch (type) {
-	case BPF_PROG_TYPE_SOCKET_FILTER:
-	case BPF_PROG_TYPE_SCHED_CLS:
-	case BPF_PROG_TYPE_SCHED_ACT:
-	case BPF_PROG_TYPE_XDP:
-	case BPF_PROG_TYPE_CGROUP_SKB:
-	case BPF_PROG_TYPE_CGROUP_SOCK:
-	case BPF_PROG_TYPE_LWT_IN:
-	case BPF_PROG_TYPE_LWT_OUT:
-	case BPF_PROG_TYPE_LWT_XMIT:
-	case BPF_PROG_TYPE_LWT_SEG6LOCAL:
-	case BPF_PROG_TYPE_SOCK_OPS:
-	case BPF_PROG_TYPE_SK_SKB:
-	case BPF_PROG_TYPE_CGROUP_DEVICE:
-	case BPF_PROG_TYPE_SK_MSG:
-	case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
-	case BPF_PROG_TYPE_LIRC_MODE2:
-	case BPF_PROG_TYPE_SK_REUSEPORT:
-	case BPF_PROG_TYPE_FLOW_DISSECTOR:
-	case BPF_PROG_TYPE_UNSPEC:
-	case BPF_PROG_TYPE_TRACEPOINT:
-	case BPF_PROG_TYPE_RAW_TRACEPOINT:
-	case BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE:
-	case BPF_PROG_TYPE_PERF_EVENT:
-	case BPF_PROG_TYPE_CGROUP_SYSCTL:
-	case BPF_PROG_TYPE_CGROUP_SOCKOPT:
-		return false;
-	case BPF_PROG_TYPE_KPROBE:
-	default:
-		return true;
-	}
-}
-
-static int bpf_object__validate(struct bpf_object *obj, bool needs_kver)
-{
-	if (needs_kver && obj->kern_version == 0) {
-		pr_warning("%s doesn't provide kernel version\n",
-			   obj->path);
-		return -LIBBPF_ERRNO__KVERSION;
-	}
-	return 0;
-}
-
 static struct bpf_object *
-__bpf_object__open(const char *path, void *obj_buf, size_t obj_buf_sz,
-		   bool needs_kver, int flags)
+__bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
+		   int flags)
 {
 	struct bpf_object *obj;
 	int err;
@@ -3617,7 +3566,6 @@ __bpf_object__open(const char *path, void *obj_buf, size_t obj_buf_sz,
 	CHECK_ERR(bpf_object__probe_caps(obj), err, out);
 	CHECK_ERR(bpf_object__elf_collect(obj, flags), err, out);
 	CHECK_ERR(bpf_object__collect_reloc(obj), err, out);
-	CHECK_ERR(bpf_object__validate(obj, needs_kver), err, out);
 
 	bpf_object__elf_finish(obj);
 	return obj;
@@ -3626,8 +3574,8 @@ __bpf_object__open(const char *path, void *obj_buf, size_t obj_buf_sz,
 	return ERR_PTR(err);
 }
 
-struct bpf_object *__bpf_object__open_xattr(struct bpf_object_open_attr *attr,
-					    int flags)
+static struct bpf_object *
+__bpf_object__open_xattr(struct bpf_object_open_attr *attr, int flags)
 {
 	/* param validation */
 	if (!attr->file)
@@ -3635,9 +3583,7 @@ struct bpf_object *__bpf_object__open_xattr(struct bpf_object_open_attr *attr,
 
 	pr_debug("loading %s\n", attr->file);
 
-	return __bpf_object__open(attr->file, NULL, 0,
-				  bpf_prog_type__needs_kver(attr->prog_type),
-				  flags);
+	return __bpf_object__open(attr->file, NULL, 0, flags);
 }
 
 struct bpf_object *bpf_object__open_xattr(struct bpf_object_open_attr *attr)
@@ -3673,7 +3619,7 @@ struct bpf_object *bpf_object__open_buffer(void *obj_buf,
 	}
 	pr_debug("loading object '%s' from buffer\n", name);
 
-	return __bpf_object__open(name, obj_buf, obj_buf_sz, true, true);
+	return __bpf_object__open(name, obj_buf, obj_buf_sz, true);
 }
 
 int bpf_object__unload(struct bpf_object *obj)

tools/lib/bpf/libbpf.h

@@ -70,8 +70,6 @@ struct bpf_object_open_attr {
 LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
 LIBBPF_API struct bpf_object *
 bpf_object__open_xattr(struct bpf_object_open_attr *attr);
-struct bpf_object *__bpf_object__open_xattr(struct bpf_object_open_attr *attr,
-					    int flags);
 LIBBPF_API struct bpf_object *bpf_object__open_buffer(void *obj_buf,
 						      size_t obj_buf_sz,
 						      const char *name);

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

@@ -49,4 +49,3 @@ int handle_uprobe_return(struct pt_regs *ctx)
 }
 
 char _license[] SEC("license") = "GPL";
-__u32 _version SEC("version") = 1;

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

@@ -99,4 +99,3 @@ int bpf_prog1(void *ctx)
 }
 
 char _license[] SEC("license") = "GPL";
-__u32 _version SEC("version") = 1; /* ignored by tracepoints, required by libbpf.a */

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

@@ -22,4 +22,3 @@ int handle_sys_nanosleep_entry(struct pt_regs *ctx)
 }
 
 char _license[] SEC("license") = "GPL";
-__u32 _version SEC("version") = 1;

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

@@ -74,4 +74,3 @@ int oncpu(struct sched_switch_args *ctx)
 }
 
 char _license[] SEC("license") = "GPL";
-__u32 _version SEC("version") = 1; /* ignored by tracepoints, required by libbpf.a */