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    SubjectRe: WARNING: kernel stack frame pointer at ffffffff82e03f40 in swapper:0 has bad value (null)
    On Mon, Dec 12, 2016 at 10:34:46PM +0100, Borislav Petkov wrote:
    > On Mon, Dec 12, 2016 at 03:16:27PM -0600, Josh Poimboeuf wrote:
    > > I still can't figure out what could cause this, nor can I recreate it.
    >
    > Want my .config?

    Yes, please.

    > > Andy, any idea? I'm trying to figure out why a stack trace of the
    > > initial task, early in start_kernel(), would show start_cpu() on the
    > > stack *twice*. The start_cpu() entry on the stack at ffffffffbce03f50
    > > is right where it's supposed to be. But then there's another
    > > start_cpu() entry at 0xffffffffbce03f48 which is pointed to by the frame
    > > pointer chain. I can't figure out where that one came from and why the
    > > stack is offset by a word, compared to all the other idle task stacks
    > > I've seen.
    >
    > Btw, why do you have:
    >
    > call 1f # put return address on stack for unwinder
    >
    > there in start_cpu() instead of
    >
    > push $start_cpu
    >
    > or so? That CALL looks strange there. If you want to put the return
    > address, just push start_cpu's address and that's it.
    >
    > Or am I missing something?

    Yeah, it's kind of obtuse.

    The problem with "push $start_cpu" is that it will show up on the stack
    trace as:

    secondary_startup_64+0x90/0x90

    instead of what you would expect:

    start_cpu+0x0/0x14

    That's because the printk '%pB' modifier is smart enough to know that
    the beginning of a function isn't a valid function call return address.
    The only way such an address could end up on the stack would be if the
    previous function made a tail call. So it shows the end of the previous
    function instead.

    That said, the code could probably be made a little clearer by changing
    "call 1f" to "push $1f" and then move the '1' label to after the lretq
    instruction, like:

    pushq $1f # put return address on stack for unwinder
    xorq %rbp, %rbp # clear frame pointer
    movq initial_code(%rip), %rax
    pushq $__KERNEL_CS # set correct cs
    pushq %rax # target address in negative space
    lretq
    1:
    ENDPROC(start_cpu)

    That shows:

    start_cpu+0x14/0x14

    Which is more accurate anyway. I'll make a patch.

    --
    Josh

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