Re: uniquely identifying KDUMP files that originate from QEMU

[Petr Tesarik <ptesarik@suse.cz>]

On Tue, 11 Nov 2014 12:22:52 +0100
Laszlo Ersek <lersek@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).

Petr Tesarik

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