netdev - [PATCH bpf v1] bpf: fix libelf endian handling in resolv

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Message-Id: <20210616092521.800788-1-Tony.Ambardar@gmail.com>
Date:   Wed, 16 Jun 2021 02:25:21 -0700
From:   Tony Ambardar <tony.ambardar@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Tony Ambardar <Tony.Ambardar@...il.com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, stable@...r.kernel.org,
        Jiri Olsa <jolsa@...nel.org>, Yonghong Song <yhs@...com>
Subject: [PATCH bpf v1] bpf: fix libelf endian handling in resolv_btfids

While patching the .BTF_ids section in vmlinux, resolve_btfids writes type
ids using host-native endianness, and relies on libelf for any required
translation when finally updating vmlinux. However, the default type of the
.BTF_ids section content is ELF_T_BYTE (i.e. unsigned char), and undergoes
no translation. This results in incorrect patched values if cross-compiling
to non-native endianness, and can manifest as kernel Oops and test failures
which are difficult to debug.

Explicitly set the type of patched data to ELF_T_WORD, allowing libelf to
transparently handle the endian conversions.

Fixes: fbbb68de80a4 ("bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object")
Cc: stable@...r.kernel.org # v5.10+
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Yonghong Song <yhs@...com>
Link: https://lore.kernel.org/bpf/CAPGftE_eY-Zdi3wBcgDfkz_iOr1KF10n=9mJHm1_a_PykcsoeA@mail.gmail.com/
Signed-off-by: Tony Ambardar <Tony.Ambardar@...il.com>
---
 tools/bpf/resolve_btfids/main.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
index d636643ddd35..f32c059fbfb4 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -649,6 +649,9 @@ static int symbols_patch(struct object *obj)
 	if (sets_patch(obj))
 		return -1;
 
+	/* Set type to ensure endian translation occurs. */
+	obj->efile.idlist->d_type = ELF_T_WORD;
+
 	elf_flagdata(obj->efile.idlist, ELF_C_SET, ELF_F_DIRTY);
 
 	err = elf_update(obj->efile.elf, ELF_C_WRITE);
-- 
2.25.1