netdev - [PATCH bpf v1 3/3] libbpf: fix native endian assumption when parsing BTF

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Date:   Sat, 19 Sep 2020 22:01:35 -0700
From:   Tony Ambardar <tony.ambardar@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     Tony Ambardar <Tony.Ambardar@...il.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-arch@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>
Subject: [PATCH bpf v1 3/3] libbpf: fix native endian assumption when parsing BTF

Code in btf__parse_raw() fails to detect raw BTF of non-native endianness
and assumes it must be ELF data, which then fails to parse as ELF and
yields a misleading error message:

  root:/# bpftool btf dump file /sys/kernel/btf/vmlinux
  libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux

For example, this could occur after cross-compiling a BTF-enabled kernel
for a target with non-native endianness, which is currently unsupported.

Check for correct endianness and emit a clearer error message:

  root:/# bpftool btf dump file /sys/kernel/btf/vmlinux
  libbpf: non-native BTF endianness is not supported

Fixes: 94a1fedd63ed ("libbpf: Add btf__parse_raw() and generic btf__parse() APIs")

Signed-off-by: Tony Ambardar <Tony.Ambardar@...il.com>
---
 tools/lib/bpf/btf.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 7dfca7016aaa..6bdbc389b493 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -659,6 +659,12 @@ struct btf *btf__parse_raw(const char *path)
 		err = -EIO;
 		goto err_out;
 	}
+	if (magic == __bswap_16(BTF_MAGIC)) {
+		/* non-native endian raw BTF */
+		pr_warn("non-native BTF endianness is not supported\n");
+		err = -LIBBPF_ERRNO__ENDIAN;
+		goto err_out;
+	}
 	if (magic != BTF_MAGIC) {
 		/* definitely not a raw BTF */
 		err = -EPROTO;
-- 
2.25.1