[PATCH] powerpc/64: poison __per_cpu_offset to catch use-before-init

[Nicholas Piggin]

If the boot CPU tries to access per-cpu data of other CPUs before
per cpu areas are set up, it will unexpectedly use offset 0.

Try to catch such accesses by poisoning the __per_cpu_offset array.

Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
With the early boot machine check handler added to 64s, this might be
worth another try. We did have a bug in mce_init() that was using
cpu_to_node() too early that would be caught by this.

Thanks,
Nick

 arch/powerpc/include/asm/percpu.h | 1 +
 arch/powerpc/kernel/paca.c        | 2 +-
 arch/powerpc/kernel/setup_64.c    | 3 ++-
 3 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/percpu.h 
b/arch/powerpc/include/asm/percpu.h
index 8e5b7d0b851c..6ca1a9fc5725 100644
--- a/arch/powerpc/include/asm/percpu.h
+++ b/arch/powerpc/include/asm/percpu.h
@@ -7,6 +7,7 @@
  * Same as asm-generic/percpu.h, except that we store the per cpu offset
  * in the paca. Based on the x86-64 implementation.
  */
+#define PER_CPU_OFFSET_POISON 0xfeeeeeeeeeeeeeeeULL
 
 #ifdef CONFIG_SMP
 
diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index be8db402e963..c33de70596e7 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/arch/powerpc/kernel/paca.c
@@ -198,7 +198,7 @@ void __init initialise_paca(struct paca_struct *new_paca, 
int cpu)
        new_paca->hw_cpu_id = 0xffff;
        new_paca->kexec_state = KEXEC_STATE_NONE;
        new_paca->__current = &init_task;
-       new_paca->data_offset = 0xfeeeeeeeeeeeeeeeULL;
+       new_paca->data_offset = PER_CPU_OFFSET_POISON;
 #ifdef CONFIG_PPC_64S_HASH_MMU
        new_paca->slb_shadow_ptr = NULL;
 #endif
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index a0dee7354fe6..039a4eac03c3 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -362,6 +362,7 @@ void __init early_setup(unsigned long dt_ptr)
         * So set up a temporary paca. It will be replaced below once we know
         * what CPU we are on.
         */
+       __per_cpu_offset[0] = 0;
        initialise_paca(&boot_paca, 0);
        fixup_boot_paca(&boot_paca);
        WARN_ON(local_paca != 0);
@@ -828,7 +829,7 @@ static __init int pcpu_cpu_to_node(int cpu)
        return early_cpu_to_node(cpu);
 }
 
-unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
+unsigned long __per_cpu_offset[NR_CPUS] __read_mostly = { [0 ... NR_CPUS-1 ] = 
PER_CPU_OFFSET_POISON };
 EXPORT_SYMBOL(__per_cpu_offset);
 
 void __init setup_per_cpu_areas(void)
-- 
2.37.2