[PATCH dwarves] btf_encoder: Fix a dwarf type DW_ATE_unsigned_1024 to btf encoding issue

From: Yonghong Song <yhs@fb.com>
To: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	<dwarves@vger.kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>, <bpf@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Eduard Zingerman <eddyz87@gmail.com>, <kernel-team@fb.com>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: [PATCH dwarves] btf_encoder: Fix a dwarf type DW_ATE_unsigned_1024 to btf encoding issue
Date: Tue, 25 Apr 2023 22:50:30 -0700	[thread overview]
Message-ID: <20230426055030.3743074-1-yhs@fb.com> (raw)

Nick Desaulniers reported an issue ([1]) where an 128-byte sized type
(DW_ATE_unsigned_1024) cannot be encoded into BTF with failure message
likes below:
  $ pahole -J reduced.o
  [2] INT DW_ATE_unsigned_1024 Error emitting BTF type
  Encountered error while encoding BTF.
See [1] for how to reproduce the issue.

The failure is due to currently BTF int type only supports upto 16
bytes (__int128) and in this case the dwarf int type is 128-byte.

The DW_ATE_unsigned_1024 is not a normal type for variable/func
declaration etc. It is used in DW_AT_location. There are two
ways to resolve this issue.
  (1). If btf encoding is expected, remove all dwarf int types
       where btf encoding will failure, e.g., non-power-of-2
       bytes, or greater than 16 bytes.
  (2). do a sanitization in btf_encoder ([2]).

This patch uses method (2) since it is a simple fix in btf_encoder.
I checked my local built vmlinux with latest
bpf-next. There is only one instance of DW_ATE_unsigned_24 (used in
DW_AT_location) so I expect irregular int types should be very rare.

  [1] https://github.com/libbpf/libbpf/pull/680
  [2] commit 7d8e829f636f ("btf_encoder: Sanitize non-regular int base type")

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 btf_encoder.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 65f6e71..1aa0ad0 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -394,7 +394,7 @@ static int32_t btf_encoder__add_base_type(struct btf_encoder *encoder, const str
 	 * these non-regular int types to avoid libbpf/kernel complaints.
 	 */
 	byte_sz = BITS_ROUNDUP_BYTES(bt->bit_size);
-	if (!byte_sz || (byte_sz & (byte_sz - 1))) {
+	if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 16) {
 		name = "__SANITIZED_FAKE_INT__";
 		byte_sz = 4;
 	}
-- 
2.34.1

