[Crash-utility] [PATCH] x86_64: do not process zero size exception stack

[HAGIO KAZUHITO(萩尾 一仁)]

Hi Alexey,

yes, I'm looking at this..

-----Original Message-----
> There is an issue with newer Linux kernel (found on 5.9.y)
> where 'bt 0' fails with:
>   crash> bt 0
>   PID: 0      TASK: ffffffff95414900  CPU: 0   COMMAND: "swapper/0"
>   bt: invalid size request: 0  type: "stack contents"
>   bt: read of stack at fffffe0000012000 failed
> 
> The reason is: readmem fails to read content of exception
> stack [4]. As you can see it has correct base: fffffe0000012000,
> but zero size, See reduced output of 'help -m':
>                   stkinfo: isize: 16384
>                            esize[7]: 4096,4096,4096,4096,0,0,0
>                            NMI_stack_index: 1
>                            exception_stacks:
>                              [0]: DOUBLEFAULT
>                              [1]: NMI
>                              [2]: DEBUG
>                              [3]: MCE
>                              [4]: (unknown)
>                              [5]: (unknown)
>                              [6]: (unknown)
>                            ebase[cpus][7]:
>    [0]: fffffe0000009000 fffffe000000b000 fffffe000000d000 fffffe000000f000
>         fffffe0000012000 0000000000000000 0000000000000000

Apparently the following patch causes this and it's in 5.10-rc ?

commit 02772fb9b68e6a72a5e17f994048df832fe2b15e
Author: Joerg Roedel <jroedel at suse.de>
Date:   Mon Sep 7 15:15:43 2020 +0200

    x86/sev-es: Allocate and map an IST stack for #VC handler

If so, it seems we should support this properly.

btw, hopefully we would like to release the new version (7.2.9) of crash
next week and are preparing, so some patches might be deferred to the next
(7.3.0 ?), especially patches for Linux 5.10.

Thanks,
Kazu

> 
> >From https://www.kernel.org/doc/Documentation/x86/kernel-stacks
> there are only 4 exception stacks for x86. So I'm not sure what
> 5th ebase ([4]) is, but its size is zero, (guard page?)
> So, solution I use here is to ignore exception stack if its size
> is zero.
> 
> Signed-off-by: Alexey Makhalov <amakhalov at vmware.com>
> ---
>  x86_64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/x86_64.c b/x86_64.c
> index fc05e8a..a8c9b2e 100644
> --- a/x86_64.c
> +++ b/x86_64.c
> @@ -5090,7 +5090,7 @@ skip_stage:
>                  	ms->stkinfo.esize[estack];
>  		console("x86_64_get_dumpfile_stack_frame: searching %s estack at %lx\n",
>  			ms->stkinfo.exception_stacks[estack], bt->stackbase);
> -		if (!(bt->stackbase))
> +		if (!(bt->stackbase) || !(ms->stkinfo.esize[estack]))
>  			goto skip_stage;
>  		bt->stackbuf = ms->irqstack;
>  		alter_stackbuf(bt);
> --
> 2.11.0
> 
> --
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