linux-kernel - Re: "relocation to !ENDBR" objtool warnings

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Message-Id: <485d1139-0c09-4d56-b076-e3e0f131b738@app.fastmail.com>
Date: Tue, 17 Dec 2024 11:37:54 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Josh Poimboeuf" <jpoimboe@...nel.org>,
 "Peter Zijlstra" <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: "relocation to !ENDBR" objtool warnings

On Tue, Dec 17, 2024, at 10:09, Arnd Bergmann wrote:
> On Tue, Dec 17, 2024, at 09:54, Arnd Bergmann wrote:
>>
>> 00000000000000f0 <stpcpy>:
>>   f0:   66 66 2e 0f 1f 84 00    data16 cs nopw 0x0(%rax,%rax,1)
>>   f7:   00 00 00 00 
>>   fb:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
>>  100:   f3 0f 1e fa             endbr64
>
>
> I had one more look at the assembler file created by gcc and found
> that this is a 16-byte nop:
>
>         .align 16
>         .globl  stpcpy
>         .type   stpcpy, @function
> stpcpy:
>         .p2align 5
>         endbr64
...
>
> No idea why it got inserted here, or why it only shows up in these
> two functions. Having a nop before the endbr is a compiler bug, right?

I confirmed that this is only a problem on gcc-14.2, both gcc-14.1
and gcc trunk (15.0 I assume) are fine according to the compiler
explorer: https://godbolt.org/z/9xf7Ph8GT

       Arnd