Hi Yanjiang,
On 05/30/2018 01:09 PM, Jin, Yanjiang wrote:
-----Original Message-----
From: Pratyush Anand [mailto:pratyush.anand@xxxxxxxxx]
Sent: 2018年5月30日 12:16
To: Jin, Yanjiang <yanjiang.jin@xxxxxxxxxxxxxxxx>
Cc: kexec@xxxxxxxxxxxxxxxxxxx; jinyanjiang@xxxxxxxxx; horms@xxxxxxxxxxxx
Subject: Re: [PATCH] arm64: update PHYS_OFFSET to conform to kernel
Hi Yanjiang,
On Wed, May 30, 2018 at 8:33 AM, Jin, Yanjiang <yanjiang.jin@xxxxxxxxxxxxxxxx>
wrote:
Hi Pratyush,
Thanks for your help! but please see my reply inline.
[...]
If an application, for example, vmcore-dmesg, wants to access the
kernel symbol which is located in the last 2M address, it would
fail with the below error:
"No program header covering vaddr 0xffff8017ffe90000 found kexec bug?"
I think, fix might not be correct.
Problem is in vmcore-dmesg and that should be fixed and not the kexec.
See here (https://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-
tools.git/tree/vmcore-dmesg/vmcore-dmesg.c?id=HEAD#n261).
Firstly, for my patch, vmcore-dmesg is just an auxiliary application to help to
reproduce this issue. The function, which is to generate vmcore, is the root cause.
...and the function which generates vmcore is not the kexec rather the secondary
kernel.
On the other hand, vmcore-dmesg is under kexec-tools, it has no a standalone
git repo. Even we want to fix vmcore-dmesg, we still need to send the patch to
kexec-tools, right?
Sure. I meant `kexec` application. We have three applications in kexec-tools.
`kexec`, `vmcore-dmesg` and `kdump`. [I hope kdump is useless and we are going
to get rid off it very soon.]
Yanjiang
How symbols are extracted from vmcore.
You do have "NUMBER(PHYS_OFFSET)=" information in vmcore.
You can probably see makedumpfile code, that how to extract
information from "NUMBER".
I have seen makedumpfile before, NUMBER(number) is just read a number
from vmcore. But as I show before, the root issue is vmcore contains a wrong
number, my patch is to fix the vmcore generating issue, we can't read vmcore at
this point since we don't have vmcore yet.
..and IIUC, you were able to reach correctly till the end of secondary kernel
where you tried vmcore-dmesg and then you had issue, right?
How did you conclude that vmcore contains wrong number? It's unlikely, but if it
does then we have problem somewhere in Linux kernel , not here.
Hi Pratyush,
I think I have found the root cause. In Linux kernel, memblock_mark_nomap() will reserve some memory ranges for EFI, such as EFI_RUNTIME_SERVICES_DATA, EFI_BOOT_SERVICES_DATA. On my environment, the first 2M memory is EFI_RUNTIME_SERVICES_DATA, so it can't be seen in kernel. We also can't set this EFI memory as "reserved", only EFI_ACPI_RECLAIM_MEMORY's memory can be set as "reserved" and seen in kernel.
So I don't think this is a kernel issue, we should fix it in kexec-tools.
Attach kernel's call stack for reference.
drivers/firmware/efi/arm-init.c
efi_init()->reserve_regions()->memblock_mark_nomap()
Hi Bhupesh,
I guess your environment has no EFI support, or the first memblock is not reserved for EFI, so you can't reproduce this issue.
Perhaps you missed reading my earlier threads on the subject of
EFI_ACPI_RECLAIM_MEMORY regions being mapped as NOMAP and how it causes
the crashkernel to panic (please go through [1]).
As of now we haven't found a acceptable-to-all solution for the issue
and it needs to be fixed in the 'kexec-tools' with a minor fix in the
kernel side as well.
So, coming back to my environment details, it has both EFI support as
well as EFI ACPI RECLAIM regions.
However we may be hitting a special case in your environment, so I
think before we can discuss your patch further (as both Pratyush and
myself have concerns with the same), would request you to share the
following:
- output of kernel dmesg with 'efi=debug' added in the bootargs (which
will help us see how the memblocks are marked at your setup - I am
specifically interested in the logs after the line 'Processing EFI
memory map'),
- if you are using a public arm64 platform maybe you can share the
CONFIG file,
- output of 'cat /proc/iomem'
[1] https://www.spinics.net/lists/arm-kernel/msg616632.html
Thanks,
Bhupesh
Have you tried to extract "PHYS_OFFSET" from vmcore either in vmcore-dmesg
or in makedumpfile and found it not matching to the value of "PHYS_OFFSET"
from first kernel?
In my understanding flow is like this:
- First kernel will have reserved area for secondary kernel, as well as for elfcore.
- First kernel will embed all the vmcore information notes into elfcore (see
crash_save_vmcoreinfo_init() -> arch_crash_save_vmcoreinfo()). Therefore, we
will have PHYS_OFFSET, kimage_voffset and VA_BITS information for first kernel
in vmcore, which is in separate memory and can be read by second kernel
- elfcore will also have notes about all the other physical memory of first kernel
which need to be copied by second kernel.
- Now when crash happens, second kernel should have all the required info for
reading symbols from first kernel's physical memory, no?
NUMBER(number) = read_vmcoreinfo_ulong(STR_NUMBER(str_number))
Yanjiang
Once you know the real PHYS_OFFSET (which could have been random if
KASLR is enabled), you can fix the problem you are seeing.
I have both validated with/without KASLR, all of them worked well after
applying my patch.
IMHO, even if that works it does not mean that its good a fix. We should try to
find root cause. Moreover, you might not have /dev/mem available for all the
configuration where KASLR is enabled.
Regards
Pratyush
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