[Crash-utility] [PATCH] Obtain KASLR offset from early S390X dumps

[Dave Anderson]

----- Original Message -----
> If the kernel crashes before vmcoreinfo initialization, there is
> no way to extract KASLR offset for such early s390 dumps.
> With a new s390 kernel patch, the KASLR offset will be stored in the lowcore
> memory during early boot and then overwritten after vmcoreinfo is initialized.
> This patch allows crash to identify the KASLR offset stored in lowcore
> memory for s390 dumps.
> 
> Signed-off-by: Mikhail Zaslonko <zaslonko at linux.ibm.com>
> ---
>  s390x.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/s390x.c b/s390x.c
> index 4a1a466..d2c6702 100644
> --- a/s390x.c
> +++ b/s390x.c
> @@ -46,6 +46,8 @@
>  
>  #define S390X_PSW_MASK_PSTATE	0x0001000000000000UL
>  
> +#define S390X_LC_VMCORE_INFO	0xe0c
> +
>  /*
>   * Flags for Region and Segment table entries.
>   */
> @@ -460,6 +462,8 @@ static void s390x_check_live(void)
>  void
>  s390x_init(int when)
>  {
> +	ulong s390x_lc_kaslr;
> +
>  	switch (when)
>  	{
>  	case SETUP_ENV:
> @@ -486,6 +490,23 @@ s390x_init(int when)
>  		machdep->verify_paddr = generic_verify_paddr;
>  		machdep->get_kvaddr_ranges = s390x_get_kvaddr_ranges;
>  		machdep->ptrs_per_pgd = PTRS_PER_PGD;
> +		if (!(kt->flags & RELOC_SET)) {
> +			/* Read the value from well-known lowcore location*/
> +			readmem(S390X_LC_VMCORE_INFO, PHYSADDR, &s390x_lc_kaslr,
> +				sizeof(s390x_lc_kaslr), "s390x_lc_kaslr",
> +				FAULT_ON_ERROR);
> +			/* Check for explicit kaslr offset flag */
> +			if (s390x_lc_kaslr & 0x1UL) {
> +				/* Drop the last bit to get an offset value */
> +				s390x_lc_kaslr &= ~(0x1UL);
> +				/* Make sure that the offset is aligned by 0x1000 */
> +				if (s390x_lc_kaslr && !(s390x_lc_kaslr & 0xfff)) {
> +					kt->relocate = s390x_lc_kaslr * (-1);
> +					kt->flags |= RELOC_SET;
> +					kt->flags2 |= KASLR;
> +				}
> +			}
> +		}
>  		break;
>  
>  	case PRE_GDB:
> --

Hi Mikhail,
 
Your patch fails on a live system that utilizes /proc/kcore as the memory source:

  # ./crash

  crash 7.2.7++
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  Copyright (C) 2005, 2011  NEC Corporation
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  and you are welcome to change it and/or distribute copies of it under
  certain conditions.  Enter "help copying" to see the conditions.
  This program has absolutely no warranty.  Enter "help warranty" for details.
 
  crash: read error: physical address: e0c  type: "s390x_lc_kaslr"
  #

That's because the newly-introduced readmem() becomes the very first memory
read access, and because you call readmem() with PHYSADDR and FAULT_ON_ERROR,
you don't allow crash to pivot from /dev/mem to /proc/kcore when it does its
first KVADDR readmem() later on during initialization:

  # ./crash -d4
  ... [ cut ] ...
     
  readmem: read_dev_mem() -> /dev/mem 
  <readmem: e0c, PHYSADDR, "s390x_lc_kaslr", 8, (FOE), 3fffbf7e848>
  <read_dev_mem: addr: e0c paddr: e0c cnt: 8>
  /dev/mem: Operation not permitted
  crash: read(/dev/mem, e0c, 8): -1 (ffffffffffffffff)
  crash: read error: physical address: e0c  type: "s390x_lc_kaslr"
  # 

Also, if there is *ever* a chance that the readmem() could fail to read
that physical address from a dumpfile, I would also suggest that you allow
it to fail quietly by changing the readmem() flag from FAULT_ON_ERROR
to QUIET|RETURN_ON_ERROR like this:

                        /* Read the value from well-known lowcore location*/
                        if (readmem(S390X_LC_VMCORE_INFO, PHYSADDR, &s390x_lc_kaslr,
                            sizeof(s390x_lc_kaslr), "s390x_lc_kaslr",
                            QUIET|RETURN_ON_ERROR)) {
                                /* Check for explicit kaslr offset flag */
                                if (s390x_lc_kaslr & 0x1UL) {
                                        /* Drop the last bit to get an offset value */
                                        s390x_lc_kaslr &= ~(0x1UL);
                                        /* Make sure that the offset is aligned by 0x1000 */
                                        if (s390x_lc_kaslr && !(s390x_lc_kaslr & 0xfff)) {
                                                kt->relocate = s390x_lc_kaslr * (-1);
                                                kt->flags |= RELOC_SET;
                                                kt->flags2 |= KASLR;
                                        }
                                }
                        }

Thanks,
  Dave