[RFC PATCH v6 04/32] bpf tools: Open eBPF object file and do basic validation

[Wang Nan <wangnan0@huawei.com>]

This patch defines basic interface of libbpf. 'struct bpf_object' will
be the handler of each object file. Its internal structure is hide to
user. eBPF object files are compiled by LLVM as ELF format. In this
patch, libelf is used to open those files, read EHDR and do basic
validation according to e_type and e_machine.

All elf related staffs are grouped together and reside in efile field of
'struct bpf_object'. bpf_object__elf_finish() is introduced to clear it.

After all eBPF programs in an object file are loaded, related ELF
information is useless. Close the object file and free those memory.

zfree() and zclose() are introduced to ensure setting NULL pointers and
negative file descriptors after resources are released.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
---
 tools/lib/bpf/libbpf.c | 158 +++++++++++++++++++++++++++++++++++++++++++++++++
 tools/lib/bpf/libbpf.h |   8 +++
 2 files changed, 166 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ded37a0..197ab5e 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11,8 +11,12 @@
 #include <stdarg.h>
 #include <string.h>
 #include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
 #include <asm/unistd.h>
 #include <linux/bpf.h>
+#include <libelf.h>
+#include <gelf.h>
 
 #include "libbpf.h"
 
@@ -52,3 +56,157 @@ void libbpf_set_print(libbpf_print_fn_t warn,
 	__pr_info = info;
 	__pr_debug = debug;
 }
+
+/* Copied from tools/perf/util/util.h */
+#ifndef zfree
+# define zfree(ptr) ({ free(*ptr); *ptr = NULL; })
+#endif
+
+#ifndef zclose
+# define zclose(fd) ({			\
+	int ___err = 0;			\
+	if ((fd) >= 0)			\
+		___err = close((fd));	\
+	fd = -1;			\
+	___err;})
+#endif
+
+#ifdef HAVE_LIBELF_MMAP_SUPPORT
+# define LIBBPF_ELF_C_READ_MMAP ELF_C_READ_MMAP
+#else
+# define LIBBPF_ELF_C_READ_MMAP ELF_C_READ
+#endif
+
+struct bpf_object {
+	/*
+	 * Information when doing elf related work. Only valid if fd
+	 * is valid.
+	 */
+	struct {
+		int fd;
+		Elf *elf;
+		GElf_Ehdr ehdr;
+	} efile;
+	char path[];
+};
+#define obj_elf_valid(o)	((o)->efile.elf)
+
+static struct bpf_object *bpf_object__new(const char *path)
+{
+	struct bpf_object *obj;
+
+	obj = calloc(1, sizeof(struct bpf_object) + strlen(path) + 1);
+	if (!obj) {
+		pr_warning("alloc memory failed for %s\n", path);
+		return NULL;
+	}
+
+	strcpy(obj->path, path);
+	obj->efile.fd = -1;
+	return obj;
+}
+
+static void bpf_object__elf_finish(struct bpf_object *obj)
+{
+	if (!obj_elf_valid(obj))
+		return;
+
+	if (obj->efile.elf) {
+		elf_end(obj->efile.elf);
+		obj->efile.elf = NULL;
+	}
+	zclose(obj->efile.fd);
+}
+
+static int bpf_object__elf_init(struct bpf_object *obj)
+{
+	int err = 0;
+	GElf_Ehdr *ep;
+
+	if (obj_elf_valid(obj)) {
+		pr_warning("elf init: internal error\n");
+		return -EEXIST;
+	}
+
+	obj->efile.fd = open(obj->path, O_RDONLY);
+	if (obj->efile.fd < 0) {
+		pr_warning("failed to open %s: %s\n", obj->path,
+				strerror(errno));
+		return -errno;
+	}
+
+	obj->efile.elf = elf_begin(obj->efile.fd,
+				 LIBBPF_ELF_C_READ_MMAP,
+				 NULL);
+	if (!obj->efile.elf) {
+		pr_warning("failed to open %s as ELF file\n",
+				obj->path);
+		err = -EINVAL;
+		goto errout;
+	}
+
+	if (!gelf_getehdr(obj->efile.elf, &obj->efile.ehdr)) {
+		pr_warning("failed to get EHDR from %s\n",
+				obj->path);
+		err = -EINVAL;
+		goto errout;
+	}
+	ep = &obj->efile.ehdr;
+
+	if ((ep->e_type != ET_REL) || (ep->e_machine != 0)) {
+		pr_warning("%s is not an eBPF object file\n",
+			obj->path);
+		err = -EINVAL;
+		goto errout;
+	}
+
+	return 0;
+errout:
+	bpf_object__elf_finish(obj);
+	return err;
+}
+
+static struct bpf_object *
+__bpf_object__open(const char *path)
+{
+	struct bpf_object *obj;
+
+	if (elf_version(EV_CURRENT) == EV_NONE) {
+		pr_warning("failed to init libelf for %s\n", path);
+		return NULL;
+	}
+
+	obj = bpf_object__new(path);
+	if (!obj)
+		return NULL;
+
+	if (bpf_object__elf_init(obj))
+		goto out;
+
+	bpf_object__elf_finish(obj);
+	return obj;
+out:
+	bpf_object__close(obj);
+	return NULL;
+}
+
+struct bpf_object *bpf_object__open(const char *path)
+{
+	/* param validation */
+	if (!path)
+		return NULL;
+
+	pr_debug("loading %s\n", path);
+
+	return __bpf_object__open(path);
+}
+
+void bpf_object__close(struct bpf_object *obj)
+{
+	if (!obj)
+		return;
+
+	bpf_object__elf_finish(obj);
+
+	free(obj);
+}
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 8d1eeba..ec3301c 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -8,6 +8,8 @@
 #ifndef __BPF_LIBBPF_H
 #define __BPF_LIBBPF_H
 
+#include <stdio.h>
+
 /*
  * In include/linux/compiler-gcc.h, __printf is defined. However
  * it should be better if libbpf.h doesn't depend on Linux header file.
@@ -20,4 +22,10 @@ void libbpf_set_print(libbpf_print_fn_t warn,
 		      libbpf_print_fn_t info,
 		      libbpf_print_fn_t debug);
 
+/* Hide internal to user */
+struct bpf_object;
+
+struct bpf_object *bpf_object__open(const char *path);
+void bpf_object__close(struct bpf_object *object);
+
 #endif
-- 
1.8.3.4