Re: 4.14.9 doesn't boot (regression)

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Dec 29, 2017 at 05:10:35PM -0700, Andy Lutomirski wrote:
> (Also, Josh, the oops code should have printed the contents of the
> struct pt_regs at the top of the DF stack.  Any idea why it didn't?)

Looking at one of the dumps:

  [  392.774879] NMI backtrace for cpu 0
  [  392.774881] CPU: 0 PID: 1 Comm: init Not tainted 4.14.9-gentoo #1
  [  392.774881] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
  [  392.774882] task: ffff8802368b8000 task.stack: ffffc9000000c000
  [  392.774885] RIP: 0010:double_fault+0x0/0x30
  [  392.774886] RSP: 0000:ffffffffff527fd0 EFLAGS: 00000086
  [  392.774887] RAX: 000000003fc00000 RBX: 0000000000000001 RCX: 00000000c0000101
  [  392.774887] RDX: 00000000ffff8802 RSI: 0000000000000000 RDI: ffffffffff527f58
  [  392.774887] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
  [  392.774888] R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff816ae726
  [  392.774888] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
  [  392.774889] FS:  0000000000000000(0000) GS:ffff88023fc00000(0000) knlGS:0000000000000000
  [  392.774889] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [  392.774890] CR2: ffffffffff526f08 CR3: 0000000235b48002 CR4: 00000000001606f0
  [  392.774892] Call Trace:
  [  392.774894]  <#DF>
  [  392.774897]  do_double_fault+0xb/0x140
  [  392.774898]  </#DF>

It should have at least printed the #DF iret frame registers, which I
recently added support for in "x86/unwinder: Handle stack overflows more
gracefully", which is in both 4.14.9 and 4.15-rc5.

I think the missing iret regs are due to a bug in show_trace_log_lvl(),
where if the unwind starts with two regs frames in a row, the second
regs don't get printed.

Alexander, would you mind reproducing again with the below patch?  It
should still fail, but this time it should hopefully show another
RIP/RSP/EFLAGS instead of the "do_double_fault+0xb/0x140" line.


diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 36b17e0febe8..39a320d077aa 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -103,6 +103,7 @@ void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 
 	unwind_start(&state, task, regs, stack);
 	stack = stack ? : get_stack_pointer(task, regs);
+	regs = unwind_get_entry_regs(&state);
 
 	/*
 	 * Iterate through the stacks, starting with the current stack pointer.
@@ -120,7 +121,7 @@ void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	 * - hardirq stack
 	 * - entry stack
 	 */
-	for (regs = NULL; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
+	for ( ; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
 		const char *stack_name;
 
 		if (get_stack_info(stack, task, &stack_info, &visit_mask)) {