[PATCH v3] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly

[PATCH v3] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly

[Saket Kumar Bhaskar]

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>
Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
---

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/

Changes since v2:
Addressed comments from Christophe:
* Modified comments.
* Adjusted the clobber list change to minimize number of small lines.

v2: https://lore.kernel.org/all/7c84fa5e24263d8247b4723118e6130d63b40912.1785131859.git.skb99@linux.ibm.com/

 arch/powerpc/kernel/irq.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index a0e8b998c9b5..cb010830c1c4 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -217,8 +217,12 @@ static __always_inline void call_do_softirq(const void *sp)
 		   [sp] "b" (sp), [offset] "i" (THREAD_SIZE - STACK_FRAME_MIN_SIZE),
 		   [callee] "i" (__do_softirq)
 		 : // Clobbers
-		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
-		   "cr7", "r0", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
+		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6", "cr7", "r0",
+		  /* 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"
 	);
 }
@@ -275,8 +279,12 @@ static __always_inline void call_do_irq(struct pt_regs *regs, void *sp)
 		   [sp] "b" (sp), [offset] "i" (THREAD_SIZE - STACK_FRAME_MIN_SIZE),
 		   [callee] "i" (__do_irq)
 		 : // Clobbers
-		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
-		   "cr7", "r0", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
+		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6", "cr7", "r0",
+		  /* r2 may be clobbered by the callee when using PCREL mode in the ELFv2 ABI. */
+#ifdef CONFIG_PPC_KERNEL_PCREL
+		   "r2",
+#endif
+		   "r4", "r5", "r6", "r7", "r8", "r9", "r10",
 		   "r11", "r12"
 	);
 }
-- 
2.54.0

Re: [PATCH v3] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly

[Hari Bathini]

On 03/08/26 10:58 am, Saket Kumar Bhaskar wrote:
> 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
> 

Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>

> Fixes: 7e3a68be42e1 ("powerpc/64: vmlinux support building with PCREL addresing")
> Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
> Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
> ---
> 
> 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/
> 
> Changes since v2:
> Addressed comments from Christophe:
> * Modified comments.
> * Adjusted the clobber list change to minimize number of small lines.
> 
> v2: https://lore.kernel.org/all/7c84fa5e24263d8247b4723118e6130d63b40912.1785131859.git.skb99@linux.ibm.com/
> 
>   arch/powerpc/kernel/irq.c | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> index a0e8b998c9b5..cb010830c1c4 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -217,8 +217,12 @@ static __always_inline void call_do_softirq(const void *sp)
>   		   [sp] "b" (sp), [offset] "i" (THREAD_SIZE - STACK_FRAME_MIN_SIZE),
>   		   [callee] "i" (__do_softirq)
>   		 : // Clobbers
> -		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
> -		   "cr7", "r0", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> +		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6", "cr7", "r0",
> +		  /* 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"
>   	);
>   }
> @@ -275,8 +279,12 @@ static __always_inline void call_do_irq(struct pt_regs *regs, void *sp)
>   		   [sp] "b" (sp), [offset] "i" (THREAD_SIZE - STACK_FRAME_MIN_SIZE),
>   		   [callee] "i" (__do_irq)
>   		 : // Clobbers
> -		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6",
> -		   "cr7", "r0", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
> +		   "lr", "xer", "ctr", "memory", "cr0", "cr1", "cr5", "cr6", "cr7", "r0",
> +		  /* r2 may be clobbered by the callee when using PCREL mode in the ELFv2 ABI. */
> +#ifdef CONFIG_PPC_KERNEL_PCREL
> +		   "r2",
> +#endif
> +		   "r4", "r5", "r6", "r7", "r8", "r9", "r10",
>   		   "r11", "r12"
>   	);
>   }

Re: [PATCH v3] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly

[Madhavan Srinivasan]

On Mon, 03 Aug 2026 10:58:43 +0530, Saket Kumar Bhaskar wrote:
> 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.
> 
> [...]

Applied to powerpc/next.

[1/1] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly
      https://git.kernel.org/powerpc/c/00be69070d91d2be978e752bb117a0a4db0e1281

cheers