[PATCH v19 10/13] arm64: kdump: add VMCOREINFO's for user-space coredump tools

Hi Takahiro,

Thanks for your reply.

On 22/06/2016:02:59:41 PM, AKASHI Takahiro wrote:

On Mon, Jun 20, 2016 at 11:02:22AM +0530, Pratyush Anand wrote:

quoted

+Atsushi

Hi Takahiro,

On 16/06/2016:11:48:28 PM, Geoff Levand wrote:

quoted

From: AKASHI Takahiro <redacted>

For the current crash utility, we need to know, at least,
  - kimage_voffset
  - PHYS_OFFSET
to handle the contents of core dump file (/proc/vmcore) correctly due to
the introduction of KASLR (CONFIG_RANDOMIZE_BASE) in v4.6.
This patch puts them as VMCOREINFO's into the file.

  - VA_BITS
is also added for makedumpfile command.

Thanks for adding them. They are quite helpful for makedumpfile as well.

quoted

More VMCOREINFO's may be added later.

Yes, we will need to pass VMCOREINFO_SYMBOL(_text) and VMCOREINFO_SYMBOL(_end)
in order to work with makedumpfile.

I know that adding those symbols is the easiest way, but
theoretically, if we know the physical address of "swapper_pg_dir",

But, we know only it's virtual address. 

instead of its virtual address, we can access all the memory pointed to
by any kernel virtual address.
How do you rationalize that we need to know "_text" and "_end"?

Well, we need some mechanism so that we can decide if an address can be
translated using linear mapping of virt_to_phys(). 

Alternatively, probably we can do like this:
-- Translate all address between "SYMBOL(swapper_pg_dir)"  and "SYMBOL(swapper_pg_dir)
 + SWAPPER_DIR_SIZE" using virt_to_phys() and now we can read values from
 dumpfile using that physical address. This way we can get PGD/PMD/PUD values.
-- PTE values may lie out side this range, however that address should still be
linearly translatable. We can use virt_to_phys() macro from them as well.

In summary, we can translate address of PGD/PMD/PUD/PTE using virt_to_phys()
and rest all can be translated using page table entries.

~Pratyush