linux-kernel - bpftool does not print full names with LLVM 17 and newer

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Date: Fri, 17 May 2024 14:50:51 -0700
From: Ivan Babrou <ivan@...udflare.com>
To: bpf <bpf@...r.kernel.org>
Cc: kernel-team <kernel-team@...udflare.com>, linux-kernel <linux-kernel@...r.kernel.org>, 
	llvm@...ts.linux.dev
Subject: bpftool does not print full names with LLVM 17 and newer

Hello,

We recently bumped LLVM used for bpftool compilation from 15 to 18 and
our alerting system notified us about some unknown bpf programs. It
turns out, the names were truncated to 15 chars, whereas before they
were longer.

After some investigation, I was able to see that the following code:

    diff --git a/src/common.c b/src/common.c
    index 958e92a..ac38506 100644
    --- a/src/common.c
    +++ b/src/common.c
    @@ -435,7 +435,9 @@ void get_prog_full_name(const struct
bpf_prog_info *prog_info, int prog_fd,
        if (!prog_btf)
            goto copy_name;

    +    printf("[0] finfo.type_id = %x\n", finfo.type_id);
        func_type = btf__type_by_id(prog_btf, finfo.type_id);
    +    printf("[1] finfo.type_id = %x\n", finfo.type_id);
        if (!func_type || !btf_is_func(func_type))
            goto copy_name;

When ran under gdb, shows:

    (gdb) b common.c:439
    Breakpoint 1 at 0x16859: file common.c, line 439.

    (gdb) r
    3403: tracing  [0] finfo.type_id = 0

    Breakpoint 1, get_prog_full_name (prog_info=0x7fffffffe160,
prog_fd=3, name_buff=0x7fffffffe030 "", buff_len=128) at common.c:439
    439        func_type = btf__type_by_id(prog_btf, finfo.type_id);
    (gdb) print finfo
    $1 = {insn_off = 0, type_id = 1547}

Notice that finfo.type_id is printed as zero, but in gdb it is in fact 1547.

Disassembly difference looks like this:

    -    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>

This can be avoided if one removes "const" during finfo initialization:

    const struct bpf_func_info finfo = {};

This seems like a pretty annoying miscompilation, and hopefully
there's a way to make clang complain about this loudly, but that's
outside of my expertise. There might be other places like this that we
just haven't noticed yet.

I can send a patch to fix this particular issue, but I'm hoping for a
more comprehensive approach from people who know better.

Thanks.