[Crash-utility] Not compatible with d52888aa2753 ("x86/mm: Move LDT remap out of KASLR region on 5-level paging")

[Dave Anderson]

----- Original Message -----
> commit d52888aa2753 ("x86/mm: Move LDT remap out of KASLR region on
> 5-level paging") changed PAGE_OFFSET from 0xffff880000000000 to
> 0xffff888000000000.
> 
> crash can't open such with CONFIG_RANDOMIZE_BASE=n:
> 
> crash: read error: kernel virtual address: ffff888ffe605024  type: "tss_struct ist array"
> 
> Edit defs.h:
> 
> -#define PAGE_OFFSET_2_6_27         0xffff880000000000
> +#define PAGE_OFFSET_2_6_27         0xffff888000000000
> 
> And this works for me.
> 
> (d52888aa2753 has also been included in 4.14.84)

Ah, so your kernel does not have "page_offset_base", which simplifies
things for KASLR kernels.  I don't have any sample vmcores that are not
configured with CONFIG_RANDOMIZE_BASE.  

Obviously just changing PAGE_OFFSET_2_6_27 in the crash utility as above
cannot be done for backwards compatibility purposes.

There would have to be 2 adjustments in the crash utility for the 
kernel change: 

  -#define __PAGE_OFFSET_BASE_L5  _AC(0xff10000000000000, UL)
  -#define __PAGE_OFFSET_BASE_L4  _AC(0xffff880000000000, UL)
  +#define __PAGE_OFFSET_BASE_L5  _AC(0xff11000000000000, UL)
  +#define __PAGE_OFFSET_BASE_L4  _AC(0xffff888000000000, UL)

where the crash utility uses PAGE_OFFSET_2_6_27 and PAGE_OFFSET_5LEVEL

  #define PAGE_OFFSET_2_6_27         0xffff880000000000
and 
  #define PAGE_OFFSET_5LEVEL         0xff10000000000000

> 
> (d52888aa2753 has also been included in 4.14.84)
>

And that's a problem.  We could gate the setting of machdep->machspec->page_offset
on LINUX(4, 20, 0), but since it's been backported, there needs to be another way
to best determine which PAGE_OFFSET value to use when KASLR is not in effect.

Dave



> 
> Thanks
> WANG Chao
> 
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