On Thu 2019-05-09 09:13:57, Steven Rostedt wrote: > On Thu, 9 May 2019 14:19:23 +0200 > Petr Mladek <pmladek@xxxxxxxx> wrote: > > > The commit 3e5903eb9cff70730 ("vsprintf: Prevent crash when dereferencing > > invalid pointers") broke boot on several architectures. The common > > pattern is that probe_kernel_read() is not working during early > > boot because userspace access framework is not ready. > > > > The check is only the best effort. Let's not rush with it during > > the early boot. > > > > Details: > > > > 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. > > > > Because it happens so early you don't get any output, just an apparently > > dead system. > > > > The stack trace (which you don't see) is something like: > > > > ... > > dump_stack+0xdc > > probe_kernel_read+0x1a4 > > check_pointer+0x58 > > string+0x3c > > vsnprintf+0x1bc > > vscnprintf+0x20 > > printk_safe_log_store+0x7c > > printk+0x40 > > dump_stack_print_info+0xbc > > dump_stack+0x8 > > probe_kernel_read+0x1a4 > > probe_kernel_read+0x19c > > check_pointer+0x58 > > string+0x3c > > vsnprintf+0x1bc > > vscnprintf+0x20 > > vprintk_store+0x6c > > vprintk_emit+0xec > > vprintk_func+0xd4 > > printk+0x40 > > cpufeatures_process_feature+0xc8 > > scan_cpufeatures_subnodes+0x380 > > of_scan_flat_dt_subnodes+0xb4 > > dt_cpu_ftrs_scan_callback+0x158 > > of_scan_flat_dt+0xf0 > > dt_cpu_ftrs_scan+0x3c > > early_init_devtree+0x360 > > early_setup+0x9c > > > > 2. Report on s390: > > > > vsnprintf invocations, are broken on s390. For example, the early boot > > output now looks like this where the first (efault) should be > > the linux_banner: > > > > [ 0.099985] (efault) > > [ 0.099985] setup: Linux is running as a z/VM guest operating system in 64-bit mode > > [ 0.100066] setup: The maximum memory size is 8192MB > > [ 0.100070] cma: Reserved 4 MiB at (efault) > > [ 0.100100] numa: NUMA mode: (efault) > > > > The reason for this, is that the code assumes that > > probe_kernel_address() works very early. This however is not true on > > at least s390. Uaccess on KERNEL_DS works only after page tables have > > been setup on s390, which happens with setup_arch()->paging_init(). > > > > Any probe_kernel_address() invocation before that will return -EFAULT. > > Hmm, this sounds to me that probe_kernel_address() is broken for these > architectures. Perhaps the system_state check should be in > probe_kernel_address() for those architectures? Yeah. Well, these problems are hard to debug. It left a dead power system with a blank screen. I am not sure if the added check is worth the pain. I hope that the check would help to debug problems. But it is yet another complexity in printk() path. I think that it is fine to keep it enabled only on the booted system for a while and get some more feedback. Best Regards, Petr