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Mon, 3 Aug 2026 06:03:50 +0000 (GMT) Message-ID: <841f9d03-06bb-4141-9bc5-a974f7cf9301@linux.ibm.com> Date: Mon, 3 Aug 2026 11:33:48 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] powerpc/irq: Fix missing r2 clobber in PCREL inline assembly To: Saket Kumar Bhaskar , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Cc: maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, chleroy@kernel.org, segher@kernel.crashing.org, venkat88@linux.ibm.com, yeswanth@linux.ibm.com References: <10fc2cda485cd22e209a31d786bed1984bdf3982.1785732393.git.skb99@linux.ibm.com> Content-Language: en-US From: Hari Bathini In-Reply-To: <10fc2cda485cd22e209a31d786bed1984bdf3982.1785732393.git.skb99@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-GUID: D5SBga4ECtjx10v6NkStWc_dNjfmwnMe X-Proofpoint-ORIG-GUID: 1gwa4IE-OvyZKfRiYiASABK50I6KHo8o X-Proofpoint-Spam-Info: AW1haW4tMjYwODAzMDA0OSBTYWx0ZWRfX0QRqPv1KHOkN DVXMhsRKceIRbsUT536mnFCcNcEGjIkq+mGG83nwS9a4u7Hy8SeojKt/sg4jir3wQ7J3xzL4A/V lOVEAG7R+zjHy0BYzwW8e6SKHg3EEXA= X-Authority-Analysis: v=2.4 cv=K8cS2SWI c=1 sm=1 tr=0 ts=6a702f4e cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=uAbxVGIbfxUO_5tXvNgY:22 a=Vt2AcnKqAAAA:8 a=mDV3o1hIAAAA:8 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=5yP404LuCDGpg1bcGhIA:9 a=QEXdDO2ut3YA:10 a=v10HlyRyNeVhbzM4Lqgd:22 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODAzMDA0OSBTYWx0ZWRfX5aLpg8SVFbB0 kWUEgfo6E4ZD8gxQS4uC1tMISRuLKa5BMcphHCb0LhsjdDRDFrCDYC0PmMTidbmGXGM1H+35FbA yGYyL0labI6NMRNi0hOPyzaLXOV/vAIbO71QkANylJOt1p5Lk1P3D2hlr1h0rebHi2BCgWrZGjy U/kiDwtYrrDOJyl16J3mWg2zAdLXHaKCiAMPYu9Q+xL6PajjI3gotn1/wB7YUJnU5JgZeYHPWdK bjqFWlnVtFQxTg3qIKjK5DXT9SctvGMaeofbjOm8mFigoD724ihd1w+Rd11PHd3JfTFThmJu1K8 rt+kEscQlXCDj03fS0jRrYiQSUSUil+Sx1OK3WJ7+CmWmWadjlLPO4XuOpwqelnVqLtDNkqkk14 sDqi+oCAMqCLChHntkJsC0utcRZ1uftQfhoHhUM5XmAoqfK2+bjTsiC6Lw+TW54Cl8JqRzL1lWw q8nVOayhPU78BJLYBaA== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-02_06,2026-07-30_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1011 spamscore=0 impostorscore=0 bulkscore=0 priorityscore=1501 lowpriorityscore=0 malwarescore=0 phishscore=0 suspectscore=0 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608030049 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 > Fixes: 7e3a68be42e1 ("powerpc/64: vmlinux support building with PCREL addresing") > Signed-off-by: Saket Kumar Bhaskar > Reviewed-by: Christophe Leroy (CS GROUP) > --- > > 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" > ); > }