Re: [BUG -tip] kmemleak and stacktrace cause page faul

[Thomas Gleixner]

On Wed, 23 Oct 2019, Cyrill Gorcunov wrote:
> On Wed, Oct 23, 2019 at 03:21:05PM +0200, Thomas Gleixner wrote:
> > Errm. estack_pages is statically initialized and it's an array of:.
> > 
> > struct estack_pages {
> >         u32     offs;
> >         u16     size;
> >         u16     type;
> > };
> > 
> > [0,2,4,5,6,8,10,12] are guard pages so 0 is not that crappy at all
> 
> Wait, Thomas, I might be wrong, but per-cpu is initialized to the pointer,
> the memory for this estack_pages has not yet been allocated, no?

static const
struct estack_pages estack_pages[CEA_ESTACK_PAGES] ____cacheline_aligned = {
        EPAGERANGE(DF),
	EPAGERANGE(NMI),
	EPAGERANGE(DB1),
	EPAGERANGE(DB),
        EPAGERANGE(MCE),
};

It's statically allocated. So it's available from the very beginning.

> The diff I made to fetch the values are
> 
> diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
> index 753b8cfe8b8a..bf0d755b6079 100644
> --- a/arch/x86/kernel/dumpstack_64.c
> +++ b/arch/x86/kernel/dumpstack_64.c
> @@ -101,8 +101,18 @@ static bool in_exception_stack(unsigned long *stack, struct stack_info *info)
>  
>  	/* Calc page offset from start of exception stacks */
>  	k = (stk - begin) >> PAGE_SHIFT;
> +
>  	/* Lookup the page descriptor */
>  	ep = &estack_pages[k];
> +
> +	printk("stk 0x%lx k %u begin 0x%lx end 0x%lx estack_pages 0x%lx ep 0x%lx\n",
> +	       stk, k, begin, end, (long)(void *)&estack_pages[0], (long)(void *)ep);
> +
> +	for (k = 0; k < CEA_ESTACK_PAGES; k++) {
> +		long v = *(long *)(void *)&estack_pages[k];
> +		printk("estack_pages[%d] = 0x%lx\n", k, v);

And as I explained to you properly decoded the values _ARE_ correct and
make sense.

> +	}
> +
>  	/* Guard page? */
>  	if (!ep->size)
>  		return false;
> 
> 
> > 
> > e.g. 0x51000 00001000
> > 
> >      bit  0-31: 00001000		Offset 0x1000: 1 Page
> >      bit 32-47: 1000			Size 0x1000:   1 Page
> >      bit 48-63: 5			Type 5: STACK_TYPE_EXCEPTION + ESTACK_DF
> > 
> > So, no. This is NOT the problem.
> 
> I drop the left of your reply. True, I agreed with anything you said.
>
> You know I didn't manage to dive more into this problem yesterday
> but if time permits I'll continue today. It is easily triggering
> under kvm (the kernel I'm building is almost without modules so
> I simply upload bzImage into the guest). FWIW, the config I'm
> using is https://gist.github.com/cyrillos/7cd5d2510a99af8ea872f07ac6f9095b

That's helpful because I enabled kmemleak and the kernel comes up just fine.

Thanks,

	tglx