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[RFC PATCH v10 07/50] bpf tools: Check endianness and make libbpf fail early

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

Libbpf doesn't magically convert missmatched endianness. Even if we swap
eBPF instructions to correct byte order, we are unable to deal with
endianness 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>
Acked-by: Alexei Starovoitov <ast@plumgrid.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 36dfbc1..15b3e82 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -192,6 +192,34 @@ errout:
 	return err;
 }
 
+static int
+bpf_object__check_endianness(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: endianness mismatch.\n");
+	return -EINVAL;
+}
+
 static struct bpf_object *
 __bpf_object__open(const char *path, void *obj_buf, size_t obj_buf_sz)
 {
@@ -208,6 +236,8 @@ __bpf_object__open(const char *path, void *obj_buf, size_t obj_buf_sz)
 
 	if (bpf_object__elf_init(obj))
 		goto out;
+	if (bpf_object__check_endianness(obj))
+		goto out;
 
 	bpf_object__elf_finish(obj);
 	return obj;
-- 
1.8.3.4

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[RFC PATCH v10 00/50] perf tools: filtering events using eBPF programs Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 18/50] bpf tools: Introduce bpf_load_program() to bpf.c Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 07/50] bpf tools: Check endianness and make libbpf fail early Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 25/50] perf tools: Call clang to compile C source to object code Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 02/50] tracing, perf: Implement BPF programs attached to uprobes Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 05/50] bpf tools: Open eBPF object file and do basic validation Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 22/50] bpf tools: Link all bpf objects onto a list Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 27/50] perf tools: Auto detecting kernel build directory Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 06/50] bpf tools: Read eBPF object from buffer Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 13/50] bpf tools: Collect relocation sections from SHT_REL sections Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 01/50] bpf: Use correct #ifdef controller for trace_call_bpf() Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 16/50] bpf tools: Create eBPF maps defined in an object file Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  [RFC PATCH v10 17/50] bpf tools: Relocate eBPF programs Wang Nan <wangnan0@huawei.com> - 2015-07-01 04:20 +0200
  Re: [RFC PATCH v10 23/50] perf tools: Make perf depend on libbpf Arnaldo Carvalho de Melo <acme@kernel.org> - 2015-07-08 15:10 +0200
  Re: [RFC PATCH v10 23/50] perf tools: Make perf depend on libbpf Arnaldo Carvalho de Melo <acme@kernel.org> - 2015-07-08 15:10 +0200

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