Re: Build breakage caused by the use of UDB

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 17, 2025 at 01:35:36PM +0100, Peter Zijlstra wrote:
> 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.

I had to (obviously) enable the RTL8192 bits, but then, yes. gcc-8 fails
to build this while gcc-10 doesn't seem to have any problems (for some
reason my random dev machine of the day doesn't seem to have gcc-9).

Let me prod at this for a bit. But also, is there a good reason you're
using this stone-age compiler? :-) And yes, its our minimum supported,
so I suppose I should go fix, but other than build testing, you really
shoulnd't be using it.