Message-ID: <09205557-a87a-3e07-8736-3e325016ca3d@molgen.mpg.de>
Date:   Thu, 29 Aug 2019 00:32:04 +0200
From:   Donald Buczek <buczek@...gen.mpg.de>
To:     Bhupesh Sharma <bhsharma@...hat.com>
Cc:     iommu <iommu@...ts.linux-foundation.org>,
        linux-pci@...r.kernel.org, x86@...nel.org,
        kexec mailing list <kexec@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        Simon Horman <horms@...ge.net.au>
Subject: Re: /proc/vmcore and wrong PAGE_OFFSET

Dear Bhupesh,

On 28.08.19 21:54, Bhupesh Sharma wrote:
> Hi Donald,
> 
> On Wed, Aug 28, 2019 at 8:38 PM Donald Buczek <buczek@...gen.mpg.de> wrote:
>>
>> On 8/20/19 11:21 PM, Donald Buczek wrote:
>>> Dear Linux folks,
>>>
>>> I'm investigating a problem, that the crash utility fails to work with our crash dumps:
>>>
>>>       buczek@...ios:/mnt$ crash vmlinux crash.vmcore
>>>       crash 7.2.6
>>>       Copyright (C) 2002-2019  Red Hat, Inc.
>>>       Copyright (C) 2004, 2005, 2006, 2010  IBM Corporation
>>>       Copyright (C) 1999-2006  Hewlett-Packard Co
>>>       Copyright (C) 2005, 2006, 2011, 2012  Fujitsu Limited
>>>       Copyright (C) 2006, 2007  VA Linux Systems Japan K.K.
>>>       Copyright (C) 2005, 2011  NEC Corporation
>>>       Copyright (C) 1999, 2002, 2007  Silicon Graphics, Inc.
>>>       Copyright (C) 1999, 2000, 2001, 2002  Mission Critical Linux, Inc.
>>>       This program is free software, covered by the GNU General Public License,
>>>       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.
>>>       GNU gdb (GDB) 7.6
>>>       Copyright (C) 2013 Free Software Foundation, Inc.
>>>       License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
>>>       This is free software: you are free to change and redistribute it.
>>>       There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
>>>       and "show warranty" for details.
>>>       This GDB was configured as "x86_64-unknown-linux-gnu"...
>>>       crash: read error: kernel virtual address: ffff89807ff77000  type: "memory section root table"
>>>
>>> The crash file is a copy of /dev/vmcore taken by a crashkernel after a sysctl-forced panic.
>>>
>>> It looks to me, that  0xffff89807ff77000 is not readable, because the virtual addresses stored in the elf header of the dump file are off by 0x0000008000000000:
>>>
>>>       buczek@...ios:/mnt$ readelf -a crash.vmcore | grep LOAD | perl -lane 'printf "%s (%016x)\n",$_,hex($F[2])-hex($F[3])'
>>>         LOAD           0x000000000000d000 0xffffffff81000000 0x000001007d000000 (fffffeff04000000)
>>>         LOAD           0x0000000001c33000 0xffff880000001000 0x0000000000001000 (ffff880000000000)
>>>         LOAD           0x0000000001cc1000 0xffff880000090000 0x0000000000090000 (ffff880000000000)
>>>         LOAD           0x0000000001cd1000 0xffff880000100000 0x0000000000100000 (ffff880000000000)
>>>         LOAD           0x0000000001cd2070 0xffff880000100070 0x0000000000100070 (ffff880000000000)
>>>         LOAD           0x0000000019bd2000 0xffff880038000000 0x0000000038000000 (ffff880000000000)
>>>         LOAD           0x000000004e6a1000 0xffff88006ffff000 0x000000006ffff000 (ffff880000000000)
>>>         LOAD           0x000000004e6a2000 0xffff880100000000 0x0000000100000000 (ffff880000000000)
>>>         LOAD           0x0000001fcda22000 0xffff882080000000 0x0000002080000000 (ffff880000000000)
>>>         LOAD           0x0000003fcd9a2000 0xffff884080000000 0x0000004080000000 (ffff880000000000)
>>>         LOAD           0x0000005fcd922000 0xffff886080000000 0x0000006080000000 (ffff880000000000)
>>>         LOAD           0x0000007fcd8a2000 0xffff888080000000 0x0000008080000000 (ffff880000000000)
>>>         LOAD           0x0000009fcd822000 0xffff88a080000000 0x000000a080000000 (ffff880000000000)
>>>         LOAD           0x000000bfcd7a2000 0xffff88c080000000 0x000000c080000000 (ffff880000000000)
>>>         LOAD           0x000000dfcd722000 0xffff88e080000000 0x000000e080000000 (ffff880000000000)
>>>         LOAD           0x000000fc4d722000 0xffff88fe00000000 0x000000fe00000000 (ffff880000000000)
>>>
>>> (Columns are File offset, Virtual Address, Physical Address and computed offset).
>>>
>>> I would expect the offset between the virtual and the physical address to be PAGE_OFFSET, which is 0xffff88800000000 on x86_64, not 0xffff880000000000. Unlike /proc/vmcore, /proc/kcore shows the same physical memory (of the last memory section above) with a correct offset:
>>>
>>>       buczek@...ios:/mnt$ sudo readelf -a /proc/kcore | grep 0x000000fe00000000 | perl -lane 'printf "%s (%016x)\n",$_,hex($F[2])-hex($F[3])'
>>>         LOAD           0x0000097e00004000 0xffff897e00000000 0x000000fe00000000 (ffff888000000000)
>>>
>>> The failing address 0xffff89807ff77000 happens to be at the end of the last memory section. It is the mem_section array, which crash wants to load and which is visible in the running system:
>>>
>>>       buczek@...ios:/mnt$ sudo gdb vmlinux /proc/kcore
>>>       [...]
>>>       (gdb) print mem_section
>>>       $1 = (struct mem_section **) 0xffff89807ff77000
>>>       (gdb) print *mem_section
>>>       $2 = (struct mem_section *) 0xffff88a07f37b000
>>>       (gdb) print **mem_section
>>>       $3 = {section_mem_map = 18446719884453740551, pageblock_flags = 0xffff88a07f36f040}
>>>
>>> I can read the same information from the crash dump, if I account for the 0x0000008000000000 error:
>>>
>>>       buczek@...ios:/mnt$ gdb vmlinux crash.vmcore
>>>       [...]
>>>       (gdb) print mem_section
>>>       $1 = (struct mem_section **) 0xffff89807ff77000
>>>       (gdb) print *mem_section
>>>       Cannot access memory at address 0xffff89807ff77000
>>>       (gdb) set $t=(struct mem_section **) ((char *)mem_section - 0x0000008000000000)
>>>       (gdb) print *$t
>>>       $2 = (struct mem_section *) 0xffff88a07f37b000
>>>       (gdb) set $s=(struct mem_section *)((char *)*$t - 0x0000008000000000 )
>>>       (gdb) print *$s
>>>       $3 = {section_mem_map = 18446719884453740551, pageblock_flags = 0xffff88a07f36f040}
>>>
>>> In the above example, the running kernel, the crashed kernel and the crashkernel are all the same 4.19.57 compilation. But I've tried with several other versions ( crashkernel 4.4, running kernel from 4.0 to linux master) with the same result.
>>>
>>> The machine in the above example has several numa nodes (this is why there are so many LOAD headers). But I've tried this with a small kvm virtual machine and got the same result.
>>>
>>>       buczek@...ios:/mnt/linux-4.19.57-286.x86_64/build$ grep RANDOMIZE_BASE .config
>>>       # CONFIG_RANDOMIZE_BASE is not set
>>>       buczek@...ios:/mnt/linux-4.19.57-286.x86_64/build$ grep SPARSEMEM .config
>>>       CONFIG_ARCH_SPARSEMEM_ENABLE=y
>>>       CONFIG_ARCH_SPARSEMEM_DEFAULT=y
>>>       CONFIG_SPARSEMEM_MANUAL=y
>>>       CONFIG_SPARSEMEM=y
>>>       CONFIG_SPARSEMEM_EXTREME=y
>>>       CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
>>>       CONFIG_SPARSEMEM_VMEMMAP=y
>>>       buczek@...ios:/mnt/linux-4.19.57-286.x86_64/build$ grep PAGE_TABLE_ISOLATION .config
>>>       CONFIG_PAGE_TABLE_ISOLATION=y
>>>
>>> Any ideas?
>>>
>>> Donald
>>
>> To answer my own question for the records:
> 
> Thanks for the update.
> 
> I think Paul (may be from your organization?) posted a similar issue
> and I had enquired about a few environment details from him for
> helping debug this issue (see <https://lkml.org/lkml/2019/8/19/938>).
> But he seems to be  OOO..

Yes, Paul is from the same organization as me. I also followed up to his email.

As I have access to the crash dump file he was talking about, I could confirm that what he reported is also a symptom of the wrong PAGE_OFFSET. So this is in fact the same issue.

>> Our kexec command line is
>>
>>       /usr/sbin/kexec -p /boot/bzImage.crash --initrd=/boot/grub/initramfs.igz --command-line="root=LABEL=root ro console=ttyS1,115200n8 console=tty0 irqpoll nr_cpus=1 reset_devices panic=5 CRASH"
>>
>> So we neither gave -s (--kexec-file-syscall) nor -a ( --kexec-syscall-auto ). For this reason, kexec used the kexec_load() syscall instead of the newer kexec_file_load syscall.
> 
> '-p' flag is for indicating a kdump operation (i.e you want to load a
> crash kernel and want to execute it if the primary kernel crashes) and
> different from the kexec load ('-l' or '-s' operation where you want
> to load and execute another kernel).

We are exclusively talking about a crash kernel.  So, we have always have -p and not -l.

-s is "Use file based syscall for kexec operation" which can be used with either, -p and -l.

>> With kexec_load(), the elf headers for the crash, which include program header for the old system ram, are not computed by the kernel, but by the userspace program from kexec-tools.
> 
> See above, kdump and kexec-load are completely different operation and
> I am not sure how using kdump options seem to help your case when
> kexec_load() / kexec_file_load() don't seem to work.

My description was unclear. This is what I think is happening:

* We call 'kexec -p /boot/bzImage.crash ...' to load a crash kernel.
* The kexec user space tool uses the system call kexec_load() for that, because we  didn't select the new method with '-s'
* For kexec_load() only, the kexec user space tool generates the additional PT_LOAD segments which cover the whole physical memory, so that the state of the crashed kernel is available to the crash kernel. This segments have the wrong vaddr.
* The crash kernel is activated by a panic()
* The crash kernel uses the segments from its elf core header to create the elf header for /proc/vmcore
* A user space script copies /proc/vmcore to permanent storage and reboots.

So we have the bad vaddr in the elf core header of the saved crash dump.

> However looking at your and Paul's original email, I can decipher that
> you are able to generate a vmcore (although an incomplete one), so I
> am pretty sure you are using the kexec -p (i.e. kdump) feature rather
> than kexec to another kernel :)

Thats right.

However, now I assume, that the vmcore is not incomplete, but that it uses bad vaddr information. See my reply to Paul:

     It is the same bug as the one described in my mail "/proc/vmcore and wrong PAGE_OFFSET". The task list can be walked if addresses are corrected by 0x0000008000000000:
     
     (gdb) set $t=&init_task
     (gdb) print $t->pid
     $1 = 0
     (gdb) set $t=container_of($t->tasks->next,struct task_struct,tasks)
     (gdb) set $t=(struct task_struct *)( (char *)$t - 0x0000008000000000)
     (gdb) print $t->pid
     $2 = 1
     (gdb) set $t=container_of($t->tasks->next,struct task_struct,tasks)
     (gdb) set $t=(struct task_struct *)( (char *)$t - 0x0000008000000000)
     (gdb) print $t->pid
     $3 = 2
     
     The debugger has wrongly mapped the physical memory at virtual 0xffff880000000000 instead of at 0xffff888000000000, because the vmcore file says so for yet unknown reasons.

>> Linux kernel commit d52888aa ("x86/mm: Move LDT remap out of KASLR region on 5-level paging") changed the base of the direct mapping from 0xffff880000000000 to 0xffff888000000000. This was merged into v4.20-rc2.
>>
>> kexec-tools, however, still has the old address hard coded:
> 
>>       buczek@...ritia:/scratch/cluster/buczek/kexec-tools (master)$ git grep X86_64_PAGE_OFFSET
>>       kexec/arch/i386/crashdump-x86.c:                        elf_info->page_offset = X86_64_PAGE_OFFSET_PRE_2_6_27;
>>       kexec/arch/i386/crashdump-x86.c:                        elf_info->page_offset = X86_64_PAGE_OFFSET;
>>       kexec/arch/i386/crashdump-x86.h:#define X86_64_PAGE_OFFSET_PRE_2_6_27   0xffff810000000000ULL
>>       kexec/arch/i386/crashdump-x86.h:#define X86_64_PAGE_OFFSET              0xffff880000000000ULL
> 
> Good catch.
> I see, while other user-space tools (for e.g. makedumpfile have
> migrated to using the available PT_LOADs for example in the
> '/proc/kcore' file (see [0] for reference) to determine the correct
> PAGE_OFFSET value, it seems kexec-tools is still using MACRO values
> for the same - which probably are not maintainable and need to be
> updated with changes in the kernel.
>
> I will try to reproduce this at my end (I think it should be easy to
> do so on Qemu) and send a kexec-tools fix shortly. I will Cc you for
> the fix patch.

Great. There's already the PRE_2_6_67 version switch in the source, so maybe that scheme can just be expanded.

> Please feel free to test the same and let me know in case you face any
> further issues.

Sure. Thank you

   Donald

> 
> Thanks,
> Bhupesh
> 

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