[RFC PATCH v4 07/29] bpf tools: Check endianess and make libbpf fail early

[Wang Nan <wangnan0@huawei.com>]

Check endianess according to EHDR. Code is taken from
tools/perf/util/symbol-elf.c.

Libbpf doesn't magically convert missmatched endianess. See discussion
on https://lkml.org/lkml/2015/5/18/650 that, even if we swap
eBPF instructions to correct byte order, we are unable to deal with
endianess in code logical generated by LLVM.

Therefore, libbpf should simply reject missmatched ELF object, and let
LLVM to create good code.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
---
 tools/lib/bpf/libbpf.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e910fb8..dbbea0c 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -169,6 +169,34 @@ errout:
 	return err;
 }
 
+static int
+bpf_object__check_endianess(struct bpf_object *obj)
+{
+	static unsigned int const endian = 1;
+
+	switch (obj->efile.ehdr.e_ident[EI_DATA]) {
+	case ELFDATA2LSB:
+		/* We are big endian, BPF obj is little endian. */
+		if (*(unsigned char const *)&endian != 1)
+			goto mismatch;
+		break;
+
+	case ELFDATA2MSB:
+		/* We are little endian, BPF obj is big endian. */
+		if (*(unsigned char const *)&endian != 0)
+			goto mismatch;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
+
+mismatch:
+	pr_warning("Error: endianess mismatch.\n");
+	return -EINVAL;
+}
+
 struct bpf_object *bpf_object__open(const char *path)
 {
 	struct bpf_object *obj;
@@ -190,6 +218,8 @@ struct bpf_object *bpf_object__open(const char *path)
 
 	if (bpf_object__elf_init(obj))
 		goto out;
+	if (bpf_object__check_endianess(obj))
+		goto out;
 
 	bpf_object__elf_finish(obj);
 	return obj;
-- 
1.8.3.4