linux-kernel - Re: [PATCH] MIPS: Work around LLVM bug when gp is used as global register variable

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Message-ID: <aYFzrjOV91rBUEB4@pie>
Date: Tue, 3 Feb 2026 04:03:58 +0000
From: Yao Zi <me@...ao.cc>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
	linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH] MIPS: Work around LLVM bug when gp is used as global
 register variable

On Mon, Feb 02, 2026 at 04:23:09PM -0700, Nathan Chancellor wrote:
> On Sun, Jan 18, 2026 at 09:02:35AM +0000, Yao Zi wrote:
> > On MIPS, __current_thread_info is defined as global register variable
> > locating in $gp, and is simply assigned with new address during kernel
> > relocation.
> > 
> > This however is broken with LLVM, which always restores $gp if it finds
> > $gp is clobbered in any form, including when intentionally through a
> > global register variable. This is against GCC's documentation[1], which
> > requires a callee-saved register used as global register variable not to
> > be restored if it's clobbered.
> > 
> > As a result, $gp will continue to point to the unrelocated kernel after
> > the epilog of relocate_kernel(), leading to an early crash in init_idle,
> > 
> > [    0.000000] CPU 0 Unable to handle kernel paging request at virtual address 0000000000000000, epc == ffffffff81afada8, ra == ffffffff81afad90
> > [    0.000000] Oops[#1]:
> > [    0.000000] CPU: 0 UID: 0 PID: 0 Comm: swapper Tainted: G        W           6.19.0-rc5-00262-gd3eeb99bbc99-dirty #188 VOLUNTARY
> > [    0.000000] Tainted: [W]=WARN
> > [    0.000000] Hardware name: loongson,loongson64v-4core-virtio
> > [    0.000000] $ 0   : 0000000000000000 0000000000000000 0000000000000001 0000000000000000
> > [    0.000000] $ 4   : ffffffff80b80ec0 ffffffff80b53d48 0000000000000000 00000000000f4240
> > [    0.000000] $ 8   : 0000000000000100 ffffffff81d82f80 ffffffff81d82f80 0000000000000001
> > [    0.000000] $12   : 0000000000000000 ffffffff81776f58 00000000000005da 0000000000000002
> > [    0.000000] $16   : ffffffff80b80e40 0000000000000000 ffffffff80b81614 9800000005dfbe80
> > [    0.000000] $20   : 00000000540000e0 ffffffff81980000 0000000000000000 ffffffff80f81c80
> > [    0.000000] $24   : 0000000000000a26 ffffffff8114fb90
> > [    0.000000] $28   : ffffffff80b50000 ffffffff80b53d40 0000000000000000 ffffffff81afad90
> > [    0.000000] Hi    : 0000000000000000
> > [    0.000000] Lo    : 0000000000000000
> > [    0.000000] epc   : ffffffff81afada8 init_idle+0x130/0x270
> > [    0.000000] ra    : ffffffff81afad90 init_idle+0x118/0x270
> > [    0.000000] Status: 540000e2	KX SX UX KERNEL EXL
> > [    0.000000] Cause : 00000008 (ExcCode 02)
> > [    0.000000] BadVA : 0000000000000000
> > [    0.000000] PrId  : 00006305 (ICT Loongson-3)
> > [    0.000000] Process swapper (pid: 0, threadinfo=(____ptrval____), task=(____ptrval____), tls=0000000000000000)
> > [    0.000000] Stack : 9800000005dfbf00 ffffffff8178e950 0000000000000000 0000000000000000
> > [    0.000000]         0000000000000000 ffffffff81970000 000000000000003f ffffffff810a6528
> > [    0.000000]         0000000000000001 9800000005dfbe80 9800000005dfbf00 ffffffff81980000
> > [    0.000000]         ffffffff810a6450 ffffffff81afb6c0 0000000000000000 ffffffff810a2258
> > [    0.000000]         ffffffff81d82ec8 ffffffff8198d010 ffffffff81b67e80 ffffffff8197dd98
> > [    0.000000]         ffffffff81d81c80 ffffffff81930000 0000000000000040 0000000000000000
> > [    0.000000]         0000000000000000 0000000000000000 0000000000000000 0000000000000000
> > [    0.000000]         0000000000000000 000000000000009e ffffffff9fc01000 0000000000000000
> > [    0.000000]         0000000000000000 0000000000000000 0000000000000000 0000000000000000
> > [    0.000000]         0000000000000000 ffffffff81ae86dc ffffffff81b3c741 0000000000000002
> > [    0.000000]         ...
> > [    0.000000] Call Trace:
> > [    0.000000] [<ffffffff81afada8>] init_idle+0x130/0x270
> > [    0.000000] [<ffffffff81afb6c0>] sched_init+0x5c8/0x6c0
> > [    0.000000] [<ffffffff81ae86dc>] start_kernel+0x27c/0x7a8
> > 
> > This bug has been reported to LLVM[2] and affects version from (at
> > least) 18 to 21. Let's work around this by using inline assembly to
> > assign $gp before a fix is widely available.
> > 
> > Cc: stable@...r.kernel.org
> > Link: https://gcc.gnu.org/onlinedocs/gcc-15.2.0/gcc/Global-Register-Variables.html # [1]
> > Link: https://github.com/llvm/llvm-project/issues/176546 # [2]
> > Signed-off-by: Yao Zi <me@...ao.cc>
> 
> Acked-by: Nathan Chancellor <nathan@...nel.org>
> 
> Hopefully the MIPS LLVM folks can look into this (even though I think
> the MIPS backend is one of the less maintained backends in LLVM).

Sigh...

> > ---
> >  arch/mips/kernel/relocate.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/arch/mips/kernel/relocate.c b/arch/mips/kernel/relocate.c
> > index 7f1c136ad850..12aa0bbdd65e 100644
> > --- a/arch/mips/kernel/relocate.c
> > +++ b/arch/mips/kernel/relocate.c
> > @@ -420,7 +420,18 @@ void *__init relocate_kernel(void)
> >  			goto out;
> >  
> >  		/* The current thread is now within the relocated image */
> > +#ifndef CONFIG_CC_IS_CLANG
> 
> I find
> 
>   #ifdef FOO
>   <FOO block>
>   #else
>   <!FOO block>
>   #endif
> 
> to be easier to read and process
> 
>   #ifndef FOO
>   <!FOO block>
>   #else
>   <FOO block>
>   #endif
> 
> but maybe that it is just personal preference.

I preferred to put Clang's workaround later, since

> >  		__current_thread_info = RELOCATED(&init_thread_union);

this simple assignment is easier to read and more clear than the inline
assembly, and matching the comment

	/* The current thread is now within the relocated image */

better. But yes, this is basically a personal preference, and I'm happy
with both. Please tell me if you do prefer the other.

> > +#else
> > +		/*
> > +		 * LLVM may wrongly restore $gp ($28) in epilog even if it's
> > +		 * intentionally modified. Work around this by using inline
> > +		 * assembly to assign $gp. $gp couldn't be listed as output or
> > +		 * clobber, or LLVM will still restore its original value.
> 
> This comment should likely include a link to the LLVM upstream report to
> make it easier to version restrict this workaround when fixed in the
> future.

Sure, will do it in v2. Thanks for the feedback!

Best regards,
Yao Zi