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

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Message-ID: <20251217124119.5832bba7@endymion>
Date: Wed, 17 Dec 2025 12:44:23 +0100
From: Jean Delvare <jdelvare@...e.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: Build breakage caused by the use of UDB

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.

-- 
Jean Delvare
SUSE L3 Support