[Crash-utility] uniquely identifying KDUMP files that originate from QEMU
HATAYAMA Daisuke
d.hatayama at jp.fujitsu.com
Thu Nov 13 01:08:57 UTC 2014
From: Dave Anderson <anderson at redhat.com>
Subject: Re: uniquely identifying KDUMP files that originate from QEMU
Date: Wed, 12 Nov 2014 09:09:34 -0500
>
>
> ----- Original Message -----
>> From: HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com>
>> To: ptesarik at suse.cz
>> Cc: lersek at redhat.com, kexec at lists.infradead.org
>> Subject: Re: uniquely identifying KDUMP files that originate from QEMU
>> Message-ID:
>> <20141112.120838.303682123986142686.d.hatayama at jp.fujitsu.com>
>> Content-Type: Text/Plain; charset=us-ascii
>>
>> From: Petr Tesarik <ptesarik at suse.cz>
>> Subject: Re: uniquely identifying KDUMP files that originate from QEMU
>> Date: Tue, 11 Nov 2014 13:09:13 +0100
>>
>> > On Tue, 11 Nov 2014 12:22:52 +0100
>> > Laszlo Ersek <lersek at redhat.com> wrote:
>> >
>> >> (Note: I'm not subscribed to either qemu-devel or the kexec list; please
>> >> keep me CC'd.)
>> >>
>> >> QEMU is able to dump the guest's memory in KDUMP format (kdump-zlib,
>> >> kdump-lzo, kdump-snappy) with the "dump-guest-memory" QMP command.
>> >>
>> >> The resultant vmcore is usually analyzed with the "crash" utility.
>> >>
>> >> The original tool producing such files is kdump. Unlike the procedure
>> >> performed by QEMU, kdump runs from *within* the guest (under a kexec'd
>> >> kdump kernel), and has more information about the original guest kernel
>> >> state (which is being dumped) than QEMU. To QEMU, the guest kernel state
>> >> is opaque.
>> >>
>> >> For this reason, the kdump preparation logic in QEMU hardcodes a number
>> >> of fields in the kdump header. The direct issue is the "phys_base"
>> >> field. Refer to dump.c, functions create_header32(), create_header64(),
>> >> and "include/sysemu/dump.h", macro PHYS_BASE (with the replacement text
>> >> "0").
>> >>
>> >> http://git.qemu.org/?p=qemu.git;a=blob;f=dump.c;h=9c7dad8f865af3b778589dd0847e450ba9a75b9d;hb=HEAD
>> >>
>> >> http://git.qemu.org/?p=qemu.git;a=blob;f=include/sysemu/dump.h;h=7e4ec5c7d96fb39c943d970d1683aa2dc171c933;hb=HEAD
>> >>
>> >> This works in most cases, because the guest Linux kernel indeed tends to
>> >> be loaded at guest-phys address 0. However, when the guest Linux kernel
>> >> is booted on top of OVMF (which has a somewhat unusual UEFI memory map),
>> >> then the guest Linux kernel is loaded at 16MB, thereby getting out of
>> >> sync with the phys_base=0 setting visible in the KDUMP header.
>> >>
>> >> This trips up the "crash" utility.
>> >>
>> >> Dave worked around the issue in "crash" for ELF format dumps -- "crash"
>> >> can identify QEMU as the originator of the vmcore by finding the QEMU
>> >> notes in the ELF vmcore. If those are present, then "crash" employs a
>> >> heuristic, probing for a phys_base up to 32MB, in 1MB steps.
>> >>
>> >> Alas, the QEMU notes are not present in the KDUMP-format vmcores that
>> >> QEMU produces (they cannot be),
>> >
>> > Why? Since KDUMP format version 4, the complete ELF notes can be stored
>> > in the file (see offset_note, size_note fields in the sub-header).
>> >
>>
>> Yes, the QEMU notes is present in kdump-compressed format. But
>> phys_base cannot be calculated only from qemu-side. We cannot do more
>> than the efforts crash utility does for workaround. So, the phys_base
>> value in kdump-sub header is now designed to have 0 now.
>>
>> Anyway, phys_base is kernel information. To make it available for qemu
>> side, there's need to prepare a mechanism for qemu to have any access
>> to it.
>>
>> One ad-hoc but simple way is to put phys_base value as part of
>> VMCOREINFO note information on kernel.
>>
>> Although there has already been a similar one in VMCOREINFO, like
>>
>> arch/x86/kernel/
>> ==
>> void arch_crash_save_vmcoreinfo(void)
>> {
>> VMCOREINFO_SYMBOL(phys_base); <---- This
>> VMCOREINFO_SYMBOL(init_level4_pgt);
>>
>> ...
>> ==
>>
>> this is meangless, because this value is a virtual address assigned to
>> phys_base symbol. To refer to the value of phys_base itself, we need
>> the phys_base value we are about to get now.
>>
>> So, instead, if we change this to save the value, not value of symbol
>> phys_base, we can get phys_base from the VMCOREINFO.
>>
>> The VMCOREINFO consists simply of string. So it's easy to search
>> vmcore for it e.g. using strings and grep like this:
>>
>> $ strings vmcore-3.10.0-121.el7.x86_64 | grep -E ".*VMCOREINFO.*" -A 100
>> VMCOREINFO
>> OSRELEASE=3.10.0-121.el7.x86_64
>> PAGESIZE=4096
>> ...
>> SYMBOL(phys_base)=ffffffff818e5010 <-- though this is address of phys_base
>> now...
>> SYMBOL(init_level4_pgt)=ffffffff818de000
>> SYMBOL(node_data)=ffffffff819f1cc0
>> LENGTH(node_data)=1024
>> CRASHTIME=1399460394
>> ...
>>
>> This should also be useful to get phys_base of 2nd kernel, which is
>> inherently relocated kernel from a vmcore generated using qemu dump.
>>
>> This is far from well-designed from qemu's point of view, but it would
>> be manually easier to get phys_base than now.
>>
>> Obviously, the VMCOREINFO is available only if CONFIG_KEXEC is
>> enabled. Other users cannot use this.
>>
>> --
>> Thanks.
>> HATAYAMA, Daisuke
>
> I agree that the actual value of phys_base should be included in the vmcoreinfo.
>
> However, it won't help in this case because the vmcoreinfo data is not
> copied into the compressed dumpfile header. The offset_vmcoreinfo and
> size_vmcoreinfo fields are zero.
Yes, so I said:
>> This is far from well-designed from qemu's point of view, but it would
>> be manually easier to get phys_base than now.
This is just an ad-hoc way.
>
> Here's an example header dump of a QEMU-generated dumpfile:
>
> crash> help -n
> makedumpfile header:
> signature: "makedumpfile"
> type: 1
> version: 1
> all_flat_data:
> num_array: 18695
> array: 7f484b760010
> file_size: 0
>
> diskdump_data:
> filename: vmcore.ovmf.rhel7.kdump-snappy
> flags: c6 (KDUMP_CMPRS_LOCAL|ERROR_EXCLUDED|LZO_SUPPORTED|SNAPPY_SUPPORTED) [FLAT]
> dfd: 3
> ofp: 3e441b1260
> machine_type: 62 (EM_X86_64)
>
> header: 1a68fe0
> signature: "KDUMP "
> header_version: 6
> utsname:
> sysname:
> nodename:
> release:
> version:
> machine: x86_64
> domainname:
> timestamp:
> tv_sec: 0
> tv_usec: 0
> status: 4 (DUMP_DH_COMPRESSED_SNAPPY)
> block_size: 4096
> sub_hdr_size: 1
> bitmap_blocks: 76
> max_mapnr: 1245184
> total_ram_blocks: 0
> device_blocks: 0
> written_blocks: 0
> current_cpu: 0
> nr_cpus: 4
> tasks[nr_cpus]: 0
> 0
> 0
> 0
>
> sub_header: 0 (n/a)
>
> sub_header_kdump: 1a69ff0
> phys_base: 0
> dump_level: 1 (0x1) (DUMP_EXCLUDE_ZERO)
> split: 0
> start_pfn: (unused)
> end_pfn: (unused)
> offset_vmcoreinfo: 0 (0x0)
> size_vmcoreinfo: 0 (0x0)
> offset_note: 4200 (0x1068)
> size_note: 3232 (0xca0)
> num_prstatus_notes: 4
> notes_buf: 1a6b000
> notes[0]: 1a6b000
> notes[1]: 1a6b164
> notes[2]: 1a6b2c8
> notes[3]: 1a6b42c
> NT_PRSTATUS_offset: 1068
> 11cc
> 1330
> 1494
> offset_eraseinfo: 0 (0x0)
> size_eraseinfo: 0 (0x0)
> start_pfn_64: (unused)
> end_pfn_64: (unused)
> max_mapnr_64: 1245184 (0x130000)
>
> data_offset: 4e000
> block_size: 4096
> block_shift: 12
> bitmap: 7f484b713010
> bitmap_len: 311296
> max_mapnr: 1245184 (0x130000)
> dumpable_bitmap: 7f484b6c6010
> byte: 0
> bit: 0
> compressed_page: 1a8c660
> curbufptr: 1a7f650
> ...
>
> Note that QEMU does add self-generated register dumps above, but the special
> "QEMU" note that is added to ELF kdumps is not included.
>
Sorry, I didn't know this, and there's no reason not to add it.
> Also note that the kernel version information is also left zero-filled.
>
This is what I intended. Retrieving data from vmcore should be done in
crash utility or makedumpfile.
> In any case, if either a QEMU note or a diskdump.data flag were added, I would
> be more than happy.
>
> Dave
The absence of QEMU note is different from my intension. This is
regression agast ELF. We must add it.
--
Thanks.
HATAYAMA, Daisuke
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