[PATCH] ARM: smp: set current pointer in assembly before jumping into C

[Karl Mehltretter <kmehltretter@gmail.com>]

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.

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 <kmehltretter@gmail.com>
---
Tested on QEMU vexpress-a15 (mainline and v6.12) and
realview-eb-mpcore with and without CONFIG_KCOV_INSTRUMENT_ALL.

The head-nommu.S change is untested because no currently reachable
Kconfig combination enables non-MMU SMP.

The redundant set_current(task) is retained to keep the fix minimal.

 arch/arm/kernel/head-nommu.S | 1 +
 arch/arm/kernel/head.S       | 1 +
 2 files changed, 2 insertions(+)

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)
--
2.43.0