Re: [PATCH v2] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

On Mon, Jul 27, 2026 at 02:09:12PM +0200, Christophe Leroy (CS GROUP) wrote:
> 
Hi Christophe, thanks for reviewing.
> 
> Le 27/07/2026 à 12:58, Saket Kumar Bhaskar a écrit :
> > In CONFIG_PPC_KERNEL_PCREL mode, r2 is no longer reserved for the TOC
> > pointer and is available as a caller-saved register [0].
> > 
> > Both call_do_irq() and call_do_softirq() use inline assembly to call
> > functions with stack switching, but fail to list r2 in their clobber
> > lists. This causes the compiler to assume r2 is preserved across these
> > calls, leading to register corruption when the called functions
> > (__do_irq and __do_softirq) clobber r2.
> > 
> > As a result of this kernel crash during interrupt handling is seen and
> > the kernel fails to boot:
> > 
> > BUG: Unable to handle kernel data access on write at 0xc000000404697638
> > Faulting instruction address: 0xc0000000000181ec
> > Oops: Kernel access of bad area, sig: 11 [#1]
> > NIP [c0000000000181ec] __do_IRQ+0x6c/0xc0
> > 
> > With older GCC, the compiler would conservatively allocate
> > callee-saved registers (like r31) for values spanning function calls,
> > accidentally avoiding the bug:
> > 
> > <__do_IRQ>:
> > 00 00 00 60 	nop
> > a6 02 08 7c 	mflr    r0
> > f8 ff e1 fb 	std     r31,-8(r1)
> > f0 ff c1 fb 	std     r30,-16(r1)
> > 2d 03 10 06 	pla     r31,53297316
> > 
> > ...
> > 
> > 3d e8 ff 4b 	bl      c0000000000165ac <__do_irq>
> > 00 00 21 e8 	ld      r1,0(r1)
> > 28 00 4d e9 	ld      r10,40(r13)
> > 40 00 21 38 	addi    r1,r1,64
> > 2a f9 aa 7f 	stdx    r29,r10,r31
> > 
> > With newer GCC 14, the compiler uses r2 for such values, exposing the
> > missing clobber specification:
> > 
> > <__do_IRQ>:
> > 00 00 00 60     nop
> > a6 02 08 7c     mflr    r0
> > f0 ff c1 fb     std     r30,-16(r1)
> > f8 ff e1 fb     std     r31,-8(r1)
> > 29 02 10 06     pla     r2,36252592     # c0000000022aadc0 <__irq_regs>
> > 
> > ...
> > 
> > 85 dc ff 4b 	bl      c000000000015ee0 <__do_irq>
> > 00 00 21 e8 	ld      r1,0(r1)
> > 28 00 2d e9 	ld      r9,40(r13)
> > 30 00 21 38 	addi    r1,r1,48
> > 2a 11 c9 7f 	stdx    r30,r9,r2
> > 
> > Fix this by adding r2 to the clobber list for both call_do_irq() and
> > call_do_softirq() when CONFIG_PPC_KERNEL_PCREL is enabled.
> > 
> > [0]: https://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg313226.html
> > 
> > Fixes: 7e3a68be42e1 ("powerpc/64: vmlinux support building with PCREL addresing")
> > Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
> > ---
> > Changes since v1:
> > Addressed comments from Segher:
> > * Modified comments to "clobber may happen"
> > 
> > v1: https://lore.kernel.org/all/dc021f42afa10396052499cbeda1772e30b7ca64.1784530547.git.skb99@linux.ibm.com/
> > 
> >   arch/powerpc/kernel/irq.c | 14 ++++++++++++--
> >   1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> > index a0e8b998c9b5..b5343cfd6c9f 100644
> > --- a/arch/powerpc/kernel/irq.c
> > +++ b/arch/powerpc/kernel/irq.c
> > @@ -218,7 +218,12 @@ static __always_inline void call_do_softirq(const void *sp)
> >   		   [callee] "i" (__do_softirq)
> >   		 : // Clobbers
> >   		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
> > -		   "cr7", "r0", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> > +		   "cr7", "r0",
> > +		  /* r2 may be clobbered by the callee when using the ELFv2 ABI */
> 
> Not sure the comment is correct. You get CONFIG_PPC64_ELF_ABI_V2 without
> CONFIG_PPC_KERNEL_PCREL.
> 
> Kconfig help presents CONFIG_PPC_KERNEL_PCREL as an ABI extension.
> 
So considering your comment and Segher's observation https://lore.kernel.org/all/al33jmn9PmMJwYlz@gate/
would be better to reword it as:
/* r2 may be clobbered by the callee when using PCREL mode in the ELFv2 ABI. */
> > +#ifdef CONFIG_PPC_KERNEL_PCREL
> > +		   "r2",
> > +#endif
> > +		   "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> >   		   "r11", "r12"
> 
> To minimise number of small lines I'd prefer something like:
> 
> 		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6", "cr7", "r0",
> #ifdef CONFIG_PPC_KERNEL_PCREL
> 		   "r2",
> #endif
> 		   "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10", "r11", "r12"
> 
> 
Noted
> >   	);
> >   }
> > @@ -276,7 +281,12 @@ static __always_inline void call_do_irq(struct pt_regs *regs, void *sp)
> >   		   [callee] "i" (__do_irq)
> >   		 : // Clobbers
> >   		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
> > -		   "cr7", "r0", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> > +		   "cr7", "r0",
> > +		  /* r2 may be clobbered by the callee when using the ELFv2 ABI */
> > +#ifdef CONFIG_PPC_KERNEL_PCREL
> > +		   "r2",
> > +#endif
> > +		   "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> 
> Same
> 
Noted
> >   		   "r11", "r12"
> >   	);
> >   }
> 
> Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
> 
> 
Thanks,
Saket