linux-kernel - Re: objtool failure caused some kernel functionality not working

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Message-ID: <14c56104-074e-4449-abd6-1290ec506390@linux.dev>
Date: Thu, 30 Jan 2025 08:30:52 -0800
From: Yonghong Song <yonghong.song@...ux.dev>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Song Liu <song@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: objtool failure caused some kernel functionality not working




On 1/30/25 12:25 AM, Josh Poimboeuf wrote:
> On Wed, Jan 29, 2025 at 11:10:14PM -0800, Yonghong Song wrote:
>> The llvm18 does not have issues. I tried to bisect what changed in llvm19 and
>> found the following llvm patch is responsible:
>>
>> https://github.com/llvm/llvm-project/pull/96089
>>
>> Basically, the compiler might be able to create a jump target which actually
>> not possible at runtime. For example, in one of examples in the above llvm patch, something
>> like
>>     if i >= 3 goto out;  /* i unsigned */
>>     switch i, label default_unreachable:
>>       case 0:  goto label1;
>>       case 1:  goto label2;
>>       case 2:  goto label3;
>>     label1: ...; return;
>>     label2: ...; return;
>>     label3: ...; return;
>>     default_unreachable:
>>
>> I think that this should be a valid code from compiler perspective.
>>
>> Can we fix objtool to handle jump target which is immediately after the func body?Thanks, Yonghong
> I actually have a fix for that, can you try this?
>
>    https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/commit/?h=objtool/core&id=fbb454b7bb39955c324693e73a5cf7e448632553

I applied the above fix and now the kernel works fine and /sys/kernel/debug/tracing/available_filter_functions
can display tracable functions properly.

The fix is in your personal branch. Any plan to send it to upstream repo?

The fix also makes sense to me. So

Tested-by: Yonghong Song <yonghong.song@...ux.dev>
Acked-by: Yonghong Song <yonghong.song@...ux.dev>