linux-kernel - [RFC PATCH v3 10/37] bpf tools: Check endianess and set swap flag according to EHDR

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Date:	Sun, 17 May 2015 10:56:35 +0000
From:	Wang Nan <wangnan0@...wei.com>
To:	<paulus@...ba.org>, <a.p.zijlstra@...llo.nl>, <mingo@...hat.com>,
	<acme@...nel.org>, <namhyung@...nel.org>, <jolsa@...nel.org>,
	<dsahern@...il.com>, <ast@...mgrid.com>, <daniel@...earbox.net>,
	<brendan.d.gregg@...il.com>, <masami.hiramatsu.pt@...achi.com>
CC:	<lizefan@...wei.com>, <linux-kernel@...r.kernel.org>,
	<pi3orama@....com>
Subject: [RFC PATCH v3 10/37] bpf tools: Check endianess and set swap flag according to EHDR

Check endianess according to EHDR to support loading eBPF objects into
big endian machines. Code is taken from tools/perf/util/symbol-elf.c.

Signed-off-by: Wang Nan <wangnan0@...wei.com>
---
 tools/lib/bpf/libbpf.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 43c16bc..a4910a8 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -69,6 +69,7 @@ void libbpf_set_print(int (*warn)(const char *format, ...),
 
 struct bpf_object {
 	char *path;
+	bool needs_swap;
 
 	/*
 	 * Information when doing elf related work. Only valid if fd
@@ -109,6 +110,7 @@ static struct bpf_object *bpf_obj_alloc(const char *path)
 	if (!obj)
 		goto out;
 
+	obj->needs_swap = false;
 	obj->elf.fd = -1;
 	return obj;
 out:
@@ -179,6 +181,31 @@ errout:
 	return err;
 }
 
+static int
+bpf_obj_swap_init(struct bpf_object *obj)
+{
+	static unsigned int const endian = 1;
+
+	obj->needs_swap = false;
+
+	switch (obj->elf.ehdr.e_ident[EI_DATA]) {
+	case ELFDATA2LSB:
+		/* We are big endian, BPF obj is little endian. */
+		if (*(unsigned char const *)&endian != 1)
+			obj->needs_swap = true;
+		return 0;
+
+	case ELFDATA2MSB:
+		/* We are little endian, BPF obj is big endian. */
+		if (*(unsigned char const *)&endian != 0)
+			obj->needs_swap = true;
+		return 0;
+
+	default:
+		return -EINVAL;
+	}
+}
+
 struct bpf_object *bpf_open_object(const char *path)
 {
 	struct bpf_object *obj;
@@ -200,6 +227,8 @@ struct bpf_object *bpf_open_object(const char *path)
 
 	if (bpf_obj_elf_init(obj))
 		goto out;
+	if (bpf_obj_swap_init(obj))
+		goto out;
 
 	return obj;
 
-- 
1.8.3.4

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