[BUG REPORT] kexec and makedumpfile can't detect PAGE_OFFSET on arm

[Wang Nan]

Hi Atsushi and Simon,

I find a problem about VMSPLIT on arm plarform, related to kexec and
makedumpfile.

When CONFIG_VMSPLIT_1G/2G is selected by kernel, PAGE_OFFSET is actually
0x40000000 or 0x80000000. However, kexec hard codes PAGE_OFFSET to
0xc0000000 (in kexec/arch/arm/crashdump-arm.h), which is incorrect in
these situations. For example, on realview-pbx board with 1G/3G VMSPLIT,
PHDRs in generated /proc/vmcore is as follow:

  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  NOTE           0x001000 0x00000000 0x00000000 0x00690 0x00690     0
  LOAD           0x002000 0xc0000000 0x00000000 0x10000000 0x10000000 RWE 0
  LOAD           0x10002000 0xe0000000 0x20000000 0x8000000 0x8000000 RWE 0
  LOAD           0x18002000 0xf0000000 0x30000000 0x10000000 0x10000000 RWE 0
  LOAD           0x28002000 0x40000000 0x80000000 0x10000000 0x10000000 RWE 0

Which should be:

  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  ...
  LOAD            ...     0x40000000 0x00000000 0x10000000 0x10000000 RWE 0
  LOAD            ...     0x60000000 0x20000000 0x8000000 0x8000000 RWE 0
  LOAD            ...     0x70000000 0x30000000 0x10000000 0x10000000 RWE 0
  LOAD            ...     0xc0000000 0x80000000 0x10000000 0x10000000 RWE 0

I don't know why crash utility can deal with it without problem, but in
makedumpfile such VMSPLIT setting causes segfault:

 $ ./makedumpfile -c -d 31 /proc/vmcore ./out -f
 The kernel version is not supported.
 The created dumpfile may be incomplete.
 Excluding unnecessary pages        : [  0.0 %] /Segmentation fault

There are many ways to deal with it, I want discuss them in the maillist and
make a decision:

 1. Kexec changes, detect PAGE_OFFSET dynamically. However, I don't know
    whether there is a reliably way for this purpose, here I suggest
    kernel to export PAGE_OFFSET through sysfs, such as
    /sys/kernel/page_offset.

 2. Or, kexec accepts PAGE_OFFSET as a command line arguments, let user
    provide correct information.

 3. Or, makedumpfile changes, don't trust EHDR anymore. Kernel should
    export PAGE_OFFSET through VMCOREINFO.

How do you feel?

Thank you!