[PATCH bpf-next v3 5/8] libbpf: Support opening bpf objects of either endianness

[Tony Ambardar <tony.ambardar@xxxxxxxxx>]

Allow bpf_object__open() to access files of either endianness, and convert
included BPF programs to native byte-order in-memory for introspection.
Loading BPF objects of non-native byte-order is still disallowed however.

Signed-off-by: Tony Ambardar <tony.ambardar@xxxxxxxxx>
---
 tools/lib/bpf/libbpf.c          | 34 +++++++++++++++++++++++++--------
 tools/lib/bpf/libbpf_internal.h | 11 +++++++++++
 2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8a0a0c1e37e1..27a5cb8ccf80 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -940,6 +940,21 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
 	return 0;
 }
 
+static void bpf_object_bswap_progs(struct bpf_object *obj)
+{
+	struct bpf_program *prog = obj->programs;
+	struct bpf_insn *insn;
+	int p, i;
+
+	for (p = 0; p < obj->nr_programs; p++, prog++) {
+		insn = prog->insns;
+		for (i = 0; i < prog->insns_cnt; i++, insn++)
+			bpf_insn_bswap(insn);
+		pr_debug("prog '%s': converted %zu insns to native byte order\n",
+			 prog->name, prog->insns_cnt);
+	}
+}
+
 static const struct btf_member *
 find_member_by_offset(const struct btf_type *t, __u32 bit_offset)
 {
@@ -1599,19 +1614,15 @@ static int bpf_object__elf_init(struct bpf_object *obj)
 	return err;
 }
 
-static int bpf_object__check_endianness(struct bpf_object *obj)
+static bool is_native_endianness(struct bpf_object *obj)
 {
 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
-	if (obj->efile.ehdr->e_ident[EI_DATA] == ELFDATA2LSB)
-		return 0;
+	return obj->efile.ehdr->e_ident[EI_DATA] == ELFDATA2LSB;
 #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-	if (obj->efile.ehdr->e_ident[EI_DATA] == ELFDATA2MSB)
-		return 0;
+	return obj->efile.ehdr->e_ident[EI_DATA] == ELFDATA2MSB;
 #else
 # error "Unrecognized __BYTE_ORDER__"
 #endif
-	pr_warn("elf: endianness mismatch in %s.\n", obj->path);
-	return -LIBBPF_ERRNO__ENDIAN;
 }
 
 static int
@@ -3953,6 +3964,10 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 		return -LIBBPF_ERRNO__FORMAT;
 	}
 
+	/* change BPF program insns to native endianness for introspection */
+	if (!is_native_endianness(obj))
+		bpf_object_bswap_progs(obj);
+
 	/* sort BPF programs by section name and in-section instruction offset
 	 * for faster search
 	 */
@@ -7993,7 +8008,6 @@ static struct bpf_object *bpf_object_open(const char *path, const void *obj_buf,
 	}
 
 	err = bpf_object__elf_init(obj);
-	err = err ? : bpf_object__check_endianness(obj);
 	err = err ? : bpf_object__elf_collect(obj);
 	err = err ? : bpf_object__collect_externs(obj);
 	err = err ? : bpf_object_fixup_btf(obj);
@@ -8498,6 +8512,10 @@ static int bpf_object_load(struct bpf_object *obj, int extra_log_level, const ch
 
 	if (obj->gen_loader)
 		bpf_gen__init(obj->gen_loader, extra_log_level, obj->nr_programs, obj->nr_maps);
+	else if (!is_native_endianness(obj)) {
+		pr_warn("object '%s' is not native endianness\n", obj->name);
+		return libbpf_err(-LIBBPF_ERRNO__ENDIAN);
+	}
 
 	err = bpf_object_prepare_token(obj);
 	err = err ? : bpf_object__probe_loading(obj);
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 6b0270c83537..020bcbf8a566 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -11,6 +11,7 @@
 
 #include <stdlib.h>
 #include <limits.h>
+#include <byteswap.h>
 #include <errno.h>
 #include <linux/err.h>
 #include <fcntl.h>
@@ -590,6 +591,16 @@ static inline bool is_ldimm64_insn(struct bpf_insn *insn)
 	return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
 }
 
+static inline void bpf_insn_bswap(struct bpf_insn *insn)
+{
+	__u8 tmp_reg = insn->dst_reg;
+
+	insn->dst_reg = insn->src_reg;
+	insn->src_reg = tmp_reg;
+	insn->off = bswap_16(insn->off);
+	insn->imm = bswap_32(insn->imm);
+}
+
 /* Unconditionally dup FD, ensuring it doesn't use [0, 2] range.
  * Original FD is not closed or altered in any other way.
  * Preserves original FD value, if it's invalid (negative).
-- 
2.34.1