linux-kernel - [PATCH 5.12 427/700] bpf: Fix libelf endian handling in resolv

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Message-Id: <20210712061021.837546242@linuxfoundation.org>
Date:   Mon, 12 Jul 2021 08:08:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Tony Ambardar <Tony.Ambardar@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jiri Olsa <jolsa@...hat.com>, Frank Eigler <fche@...hat.com>,
        Mark Wielaard <mark@...mp.org>, Jiri Olsa <jolsa@...nel.org>,
        Yonghong Song <yhs@...com>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.12 427/700] bpf: Fix libelf endian handling in resolv_btfids

From: Tony Ambardar <tony.ambardar@...il.com>

[ Upstream commit 61e8aeda9398925f8c6fc290585bdd9727d154c4 ]

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")
Signed-off-by: Tony Ambardar <Tony.Ambardar@...il.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Jiri Olsa <jolsa@...hat.com>
Cc: Frank Eigler <fche@...hat.com>
Cc: Mark Wielaard <mark@...mp.org>
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 [1]
Link: https://lore.kernel.org/bpf/20210618061404.818569-1-Tony.Ambardar@gmail.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 80d966cfcaa1..7ce4558a932f 100644
--- a/tools/bpf/resolve_btfids/main.c
+++ b/tools/bpf/resolve_btfids/main.c
@@ -656,6 +656,9 @@ 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);
-- 
2.30.2