linux-kernel - Re: Build breakage caused by the use of UDB

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Message-ID: <20251217123536.GE3707891@noisy.programming.kicks-ass.net>
Date: Wed, 17 Dec 2025 13:35:36 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Jean Delvare <jdelvare@...e.de>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: Re: Build breakage caused by the use of UDB

On Wed, Dec 17, 2025 at 12:44:23PM +0100, Jean Delvare wrote:
> Hi Peter,
> 
> Kernel v6.18.1 doesn't build on x86_64 for me. The build failure is:
> 
> make[1]: Entering directory '/home/khali/src/linux-6.18/drivers/net/wireless/realtek/rtlwifi/rtl8192c'
>   CC [M]  main.o
>   CC [M]  dm_common.o
>   CC [M]  fw_common.o
>   CC [M]  phy_common.o
>   CHECK   main.c
> fw_common.c: Assembler messages:
> fw_common.c:416: Error: unknown pseudo-op: `.byte0x0f'
> fw_common.c:391: Error: unknown pseudo-op: `.byte0x0f'
> make[3]: *** [/home/khali/src/linux-6.18/scripts/Makefile.build:287: fw_common.o] Error 1
> make[3]: *** Waiting for unfinished jobs....
>   CHECK   dm_common.c
> phy_common.c: Assembler messages:
> phy_common.c:58: Error: unknown pseudo-op: `.byte0x0f'
> phy_common.c:67: Error: unknown pseudo-op: `.byte0x0f'
> ../wifi.h:3033: Error: unknown pseudo-op: `.byte0x0f'
> ../wifi.h:3033: Error: unknown pseudo-op: `.byte0x0f'
> phy_common.c:807: Error: unknown pseudo-op: `.byte0x0f'
> phy_common.c:714: Error: unknown pseudo-op: `.byte0x0f'
> make[3]: *** [/home/khali/src/linux-6.18/scripts/Makefile.build:287: phy_common.o] Error 1
> make[2]: *** [/home/khali/src/linux-6.18/Makefile:2010: .] Error 2
> make[1]: *** [/home/khali/src/linux-6.18/Makefile:248: __sub-make] Error 2
> make[1]: Leaving directory '/home/khali/src/linux-6.18/drivers/net/wireless/realtek/rtlwifi/rtl8192c'
> make: *** [Makefile:248: __sub-make] Error 2
> 
> I bisected it down to:
> 
> commit 85a2d4a890dce3cfc9c14aa91afc3dd7af8e3bf5
> Author: Peter Zijlstra
> Date:   Mon Sep 1 12:49:58 2025 +0200
> 
>     x86,ibt: Use UDB instead of 0xEA
> 
> Reverting this commit allows me to build v6.18.1.
> 
> I must confess this is all way beyond me and I have no idea how this
> change can cause such a build failure, but it does. If it matters, my
> compiler is gcc 8.2.1.

Well, that is somewhat unexpected. None of the build robots fingered
this. Is there a particular .config I should try?

I don't seem to have 8.2.1 at hand, but I'll try with 8.3.0.