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    SubjectRe: 4.14.9 doesn't boot (regression)
    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)) {
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