linux-kernel - [PATCH] bpftool: un-const bpf_func_info to fix it for llvm 17 and newer

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Date: Mon, 20 May 2024 15:51:49 -0700
From: Ivan Babrou <ivan@...udflare.com>
To: bpf@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...udflare.com,
	Ivan Babrou <ivan@...udflare.com>,
	Quentin Monnet <quentin@...valent.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Tom Rix <trix@...hat.com>,
	Raman Shukhau <ramasha@...com>
Subject: [PATCH] bpftool: un-const bpf_func_info to fix it for llvm 17 and newer

LLVM 17 started treating const structs as constants:

* https://github.com/llvm/llvm-project/commit/0b2d5b967d98

Combined with pointer laundering via ptr_to_u64, which takes a const ptr,
but in reality treats the underlying memory as mutable, this makes clang
always pass zero to btf__type_by_id, which breaks full name resolution.

Disassembly before (LLVM 16) and after (LLVM 17):

    -    8b 75 cc                 mov    -0x34(%rbp),%esi
    -    e8 47 8d 02 00           call   3f5b0 <btf__type_by_id>
    +    31 f6                    xor    %esi,%esi
    +    e8 a9 8c 02 00           call   3f510 <btf__type_by_id>

It's a bigger project to fix this properly (and a question whether LLVM
itself should detect this), but for right now let's just fix bpftool.

For more information, see this thread in bpf mailing list:

* https://lore.kernel.org/bpf/CABWYdi0ymezpYsQsPv7qzpx2fWuTkoD1-wG1eT-9x-TSREFrQg@mail.gmail.com/T/

Fixes: b662000aff84 ("bpftool: Adding support for BTF program names")
Signed-off-by: Ivan Babrou <ivan@...udflare.com>
---
 tools/bpf/bpftool/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/common.c b/tools/bpf/bpftool/common.c
index cc6e6aae2447..6d8bbc3ec603 100644
--- a/tools/bpf/bpftool/common.c
+++ b/tools/bpf/bpftool/common.c
@@ -338,7 +338,7 @@ void get_prog_full_name(const struct bpf_prog_info *prog_info, int prog_fd,
 {
 	const char *prog_name = prog_info->name;
 	const struct btf_type *func_type;
-	const struct bpf_func_info finfo = {};
+	struct bpf_func_info finfo = {};
 	struct bpf_prog_info info = {};
 	__u32 info_len = sizeof(info);
 	struct btf *prog_btf = NULL;
-- 
2.44.0