linux-kernel - Re: [PATCH v3 32/59] objtool: Allow STT_NOTYPE -> STT

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Message-ID: <Yyw9Pvo7/7YWoKds@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Sep 2022 12:47:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v3 32/59] objtool: Allow STT_NOTYPE -> STT_FUNC+0
 tail-calls

On Thu, Sep 22, 2022 at 12:29:58PM +0200, Peter Zijlstra wrote:
> I managed to reproduce with 12.2.0 -- my 12.1.0 compiler didn't
> reproduce.
> 
> $ ./scripts/objdump-func vmlinux.o ftrace_replace_code

> 0000 0000000000012a83 <ftrace_replace_code.cold>:
> 0000    12a83:	48 89 de             	mov    %rbx,%rsi
> 0003    12a86:	89 c7                	mov    %eax,%edi
> 0005    12a88:	5b                   	pop    %rbx
> 0006    12a89:	5d                   	pop    %rbp
> 0007    12a8a:	41 5c                	pop    %r12
> 0009    12a8c:	41 5d                	pop    %r13
> 000b    12a8e:	41 5e                	pop    %r14
> 000d    12a90:	e9 62 fd ff ff       	jmp    127f7 <ftrace_bug>
> 
> 
> Seems to suggest objtool is actually right; I cannot find a reference to
> that cold symbol.

Ohhhh, ftrace_replace_cold is a weak function, so it could be the
original weak symbol had a reference to the cold thing.

We have some code to deal with crap like that, lemme try and figure out
what went wrong.