powerpc/64: Make stack tracing work during very early boot

If we try to stack trace very early during boot, either due to a
WARN/BUG or manual dump_stack(), we will oops in
valid_emergency_stack() when we try to dereference the paca_ptrs
array.

The fix is simple, we just return false if paca_ptrs isn't allocated
yet. The stack pointer definitely isn't part of any emergency stack
because we haven't allocated any yet.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210202130207.1303975-1-mpe@ellerman.id.au

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 8520ed5ae144dc1a982fc6620f648a3ec2dbcbbd..e296440e9d16aabe5dd5818eb799a72e29a35b01 100644 (file)
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -2047,6 +2047,9 @@ static inline int valid_emergency_stack(unsigned long sp, struct task_struct *p,
        unsigned long stack_page;
        unsigned long cpu = task_cpu(p);
 
+       if (!paca_ptrs)
+               return 0;
+
        stack_page = (unsigned long)paca_ptrs[cpu]->emergency_sp - THREAD_SIZE;
        if (sp >= stack_page && sp <= stack_page + THREAD_SIZE - nbytes)
                return 1;