On (05/09/19 14:19), Petr Mladek wrote: > 1. Report on Power: > > Kernel crashes very early during boot with with CONFIG_PPC_KUAP and > CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG > > The problem is the combination of some new code called via printk(), > check_pointer() which calls probe_kernel_read(). That then calls > allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early > (before we've patched features). With the JUMP_LABEL debug enabled that > causes us to call printk() & dump_stack() and we end up recursing and > overflowing the stack. Hmm... hmm... PPC does an .opd-based symbol dereference, which eventually probe_kernel_read()-s. So early printk(%pS) will do printk(%pS) dereference_function_descriptor() probe_kernel_address() dump_stack() printk(%pS) dereference_function_descriptor() probe_kernel_address() dump_stack() printk(%pS) ... I'd say... that it's not vsprintf that we want to fix, it's the idea that probe_kernel_address() can dump_stack() on any platform. On some archs probe_kernel_address()->dump_stack() is going nowhere: dump_stack() does probe_kernel_address(), which calls dump_stack(), which calls printk(%pS)->probe_kernel_address() again and again, and again. -ss
