netdev - [PATCH bpf v2] bpf: fix libelf endian handling in resolv

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Message-Id: <20210618061404.818569-1-Tony.Ambardar@gmail.com>
Date:   Thu, 17 Jun 2021 23:14:04 -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, linux-mips@...r.kernel.org,
        stable@...r.kernel.org, Frank Eigler <fche@...hat.com>,
        Mark Wielaard <mark@...mp.org>, Jiri Olsa <jolsa@...nel.org>,
        Yonghong Song <yhs@...com>, Jiri Olsa <jolsa@...hat.com>
Subject: [PATCH bpf v2] bpf: fix libelf endian handling in resolv_btfids

The vmlinux ".BTF_ids" ELF section is declared in btf_ids.h to hold a list
of zero-filled BTF IDs, which is then patched at link-time with correct
values by resolv_btfids. The section is flagged as "allocable" to preclude
compression, but notably the section contents (BTF IDs) are untyped.

When patching the BTF IDs, resolve_btfids writes in host-native endianness
and relies on libelf for any required translation on reading and updating
vmlinux. However, since the type of the .BTF_ids section content defaults
to ELF_T_BYTE (i.e. unsigned char), no translation occurs. This results in
incorrect patched values when cross-compiling to non-native endianness,
and can manifest as kernel Oops and test failures which are difficult to
troubleshoot [1].

Explicitly set the type of patched data to ELF_T_WORD, the architecture-
neutral ELF type corresponding to the u32 BTF IDs. This enables libelf to
transparently perform any needed endian conversions.

Fixes: fbbb68de80a4 ("bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object")
Cc: stable@...r.kernel.org # v5.10+
Cc: Frank Eigler <fche@...hat.com>
Cc: Mark Wielaard <mark@...mp.org>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: Yonghong Song <yhs@...com>
Link: [1] https://lore.kernel.org/bpf/CAPGftE_eY-Zdi3wBcgDfkz_iOr1KF10n=9mJHm1_a_PykcsoeA@mail.gmail.com/
Signed-off-by: Tony Ambardar <Tony.Ambardar@...il.com>
Acked-by: Jiri Olsa <jolsa@...hat.com>
---
v1 -> v2:
 * add context and elaborate on commit message per request
 * include ACK from Jiri Olsa
---
 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