Currently restore_user_regs deallocates the SVC stack early in its execution and relies on no exception being taken between the deallocation and the registers being restored. The introduction of a default FIQ handler that also uses the SVC stack breaks this assumption and can result in corrupted register state.
This patch works around the problem by removing the early stack deallocation and using r2 as a temporary instead. I have not found a way to do this without introducing an extra mov instruction to the macro.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org ---
Notes: I have recently started to hook up the PMU via FIQ (although its slightly hacky at present) and was seeing random userspace SEGVs when perf was running (after ~100,000 or so FIQs).
Instrumenting the code eventually revealed that in almost all cases the last FIQ handler to run prior the SEGV had interrupted ret_to_user_from_irq or ret_fast_syscall. Very occasionally it was in the fault handling code (because that code runs as part of SEGV handling and the PMU is instrumenting that too).
No SEGV problems have been observed since fixing the issue. This version of the patch has seen >7M FIQs and an older version (based on cpsid f) ran overnight.
arch/arm/kernel/entry-header.S | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S index 4176df721bf0..1a0045abead7 100644 --- a/arch/arm/kernel/entry-header.S +++ b/arch/arm/kernel/entry-header.S @@ -253,21 +253,22 @@ .endm
.macro restore_user_regs, fast = 0, offset = 0 - ldr r1, [sp, #\offset + S_PSR] @ get calling cpsr - ldr lr, [sp, #\offset + S_PC]! @ get pc + mov r2, sp + ldr r1, [r2, #\offset + S_PSR] @ get calling cpsr + ldr lr, [r2, #\offset + S_PC]! @ get pc msr spsr_cxsf, r1 @ save in spsr_svc #if defined(CONFIG_CPU_V6) || defined(CONFIG_CPU_32v6K) @ We must avoid clrex due to Cortex-A15 erratum #830321 - strex r1, r2, [sp] @ clear the exclusive monitor + strex r1, r2, [r2] @ clear the exclusive monitor #endif .if \fast - ldmdb sp, {r1 - lr}^ @ get calling r1 - lr + ldmdb r2, {r1 - lr}^ @ get calling r1 - lr .else - ldmdb sp, {r0 - lr}^ @ get calling r0 - lr + ldmdb r2, {r0 - lr}^ @ get calling r0 - lr .endif mov r0, r0 @ ARMv5T and earlier require a nop @ after ldm {}^ - add sp, sp, #S_FRAME_SIZE - S_PC + add sp, sp, #\offset + S_FRAME_SIZE movs pc, lr @ return & move spsr_svc into cpsr .endm
-- 1.9.3