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[2a02:3100:b118:f401:4d:3df1:8834:8499]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47fd41d1956sm17607243f8f.6.2026.08.01.09.35.30 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Sat, 01 Aug 2026 09:35:31 -0700 (PDT) From: Karl Mehltretter To: Russell King Cc: Karl Mehltretter , Ard Biesheuvel , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH v2] ARM: smp: set current pointer in assembly before jumping into C Date: Sat, 1 Aug 2026 18:35:20 +0200 Message-Id: <20260801163520.74751-1-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260801_093534_591571_9776199B X-CRM114-Status: GOOD ( 16.33 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org With CONFIG_KCOV_INSTRUMENT_ALL=y, secondary CPUs fail to come online on ARMv6K/ARMv7 SMP: CPU1: failed to boot: -38 The compiler inserts a call to __sanitizer_cov_trace_pc() at the entry of secondary_start_kernel(), before set_current(task). The KCOV hook dereferences current while TPIDRURO is still uninitialized, causing a data abort before cpu_init() has installed the exception stacks. The secondary CPU consequently never completes startup. Initialize current in the secondary assembly handoff, as __mmap_switched already does for the boot CPU. With current set in assembly, the set_current() call in secondary_start_kernel() is redundant. Drop it together with the function and the task argument. The hotplug resuscitate path in arch_cpu_idle_dead() needs no hand-off either, as smp_ops.cpu_die() can only return if the CPU kept its register state, TPIDRURO included. Fixes: 50596b7559bf ("ARM: smp: Store current pointer in TPIDRURO register if available") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-fable-5 Signed-off-by: Karl Mehltretter --- v2: drop the redundant set_current() call and helper (Ard), along with the now-unused task argument to secondary_start_kernel() v1: https://lore.kernel.org/r/20260718091600.92021-1-kmehltretter@gmail.com/ Tested on QEMU vexpress-a15 (v6.16) with CONFIG_KCOV_INSTRUMENT_ALL=y, covering SMP bringup and 10 CPU1 offline/online cycles through the arch_cpu_idle_dead() resuscitate path. The head-nommu.S change is untested because no currently reachable Kconfig combination enables non-MMU SMP. arch/arm/include/asm/smp.h | 2 +- arch/arm/kernel/head-nommu.S | 1 + arch/arm/kernel/head.S | 1 + arch/arm/kernel/smp.c | 15 ++------------- 4 files changed, 5 insertions(+), 14 deletions(-) diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h index 8c05a7f374d8..84aa6a663e81 100644 --- a/arch/arm/include/asm/smp.h +++ b/arch/arm/include/asm/smp.h @@ -43,7 +43,7 @@ extern void set_smp_ipi_range(int ipi_base, int nr_ipi); * Called from platform specific assembly code, this is the * secondary CPU entry point. */ -asmlinkage void secondary_start_kernel(struct task_struct *task); +asmlinkage void secondary_start_kernel(void); /* diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S index b9d6818f1ee1..f80bbfbd3a6f 100644 --- a/arch/arm/kernel/head-nommu.S +++ b/arch/arm/kernel/head-nommu.S @@ -116,6 +116,7 @@ ENTRY(secondary_startup) ldr r7, __secondary_data @ reload r7 ldr sp, [r7, #12] @ set up the stack pointer ldr r0, [r7, #16] @ set up task pointer + set_current r0, r1 mov fp, #0 b secondary_start_kernel ENDPROC(secondary_startup) diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S index f22c50d4bd41..a22403a629ef 100644 --- a/arch/arm/kernel/head.S +++ b/arch/arm/kernel/head.S @@ -442,6 +442,7 @@ ENTRY(__secondary_switched) adr_l r7, secondary_data + 12 @ get secondary_data.stack ldr sp, [r7] ldr r0, [r7, #4] @ get secondary_data.task + set_current r0, r1 mov fp, #0 b secondary_start_kernel ENDPROC(__secondary_switched) diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c index 50999886a8b5..e30206365877 100644 --- a/arch/arm/kernel/smp.c +++ b/arch/arm/kernel/smp.c @@ -371,12 +371,9 @@ void __noreturn arch_cpu_idle_dead(void) */ __asm__("mov sp, %0\n" " mov fp, #0\n" - " mov r0, %1\n" " b secondary_start_kernel" : - : "r" (task_stack_page(current) + THREAD_SIZE - 8), - "r" (current) - : "r0"); + : "r" (task_stack_page(current) + THREAD_SIZE - 8)); unreachable(); } @@ -397,23 +394,15 @@ static void smp_store_cpu_info(unsigned int cpuid) check_cpu_icache_size(cpuid); } -static void set_current(struct task_struct *cur) -{ - /* Set TPIDRURO */ - asm("mcr p15, 0, %0, c13, c0, 3" :: "r"(cur) : "memory"); -} - /* * This is the secondary CPU boot entry. We're using this CPUs * idle thread stack, but a set of temporary page tables. */ -asmlinkage void secondary_start_kernel(struct task_struct *task) +asmlinkage void secondary_start_kernel(void) { struct mm_struct *mm = &init_mm; unsigned int cpu; - set_current(task); - secondary_biglittle_init(); /* -- 2.43.0