On Wed, Nov 09, 2016 at 11:37:35AM +0000, Daniel P. Berrange wrote: > On Wed, Nov 09, 2016 at 12:26:17PM +0100, Laszlo Ersek wrote: > > On 11/09/16 11:40, Andrew Jones wrote: > > > On Wed, Nov 09, 2016 at 11:01:46AM +0800, Dave Young wrote: > > >> Hi, > > >> > > >> Latest linux kernel enabled kaslr to randomiz phys/virt memory > > >> addresses, we had some effort to support kexec/kdump so that crash > > >> utility can still works in case crashed kernel has kaslr enabled. > > >> > > >> But according to Dave Anderson virsh dump does not work, quoted messages > > >> from Dave below: > > >> > > >> """ > > >> with virsh dump, there's no way of even knowing that KASLR > > >> has randomized the kernel __START_KERNEL_map region, because there is no > > >> virtual address information -- e.g., like "SYMBOL(_stext)" in the kdump > > >> vmcoreinfo data to compare against the vmlinux file symbol value. > > >> Unless virsh dump can export some basic virtual memory data, which > > >> they say it can't, I don't see how KASLR can ever be supported. > > >> """ > > >> > > >> I assume virsh dump is using qemu guest memory dump facility so it > > >> should be first addressed in qemu. Thus post this query to qemu devel > > >> list. If this is not correct please let me know. > > >> > > >> Could you qemu dump people make it work? Or we can not support virt dump > > >> as long as KASLR being enabled. Latest Fedora kernel has enabled it in > > >> x86_64. > > >> > > > > > > When the -kernel command line option is used, then it may be possible > > > to extract some information that could be used to supplement the memory > > > dump that dump-guest-memory provides. However, that would be a specific > > > use. In general, QEMU knows nothing about the guest kernel. It doesn't > > > know where it is in the disk image, and it doesn't even know if it's > > > Linux. > > > > > > Is there anything a guest userspace application could probe from e.g. > > > /proc that would work? If so, then the guest agent could gain a new > > > feature providing that. > > > > I fully agree. This is exactly what I suggested too, independently, in > > the downstream thread, before arriving at this upstream thread. Let me > > quote that email: > > > > On 11/09/16 12:09, Laszlo Ersek wrote: > > > [...] the dump-guest-memory QEMU command supports an option called > > > "paging". Here's its documentation, from the "qapi-schema.json" source > > > file: > > > > > >> # @paging: if true, do paging to get guest's memory mapping. This allows > > >> # using gdb to process the core file. > > >> # > > >> # IMPORTANT: this option can make QEMU allocate several > > >> gigabytes > > >> # of RAM. This can happen for a large guest, or a > > >> # malicious guest pretending to be large. > > >> # > > >> # Also, paging=true has the following limitations: > > >> # > > >> # 1. The guest may be in a catastrophic state or can have > > >> corrupted > > >> # memory, which cannot be trusted > > >> # 2. The guest can be in real-mode even if paging is > > >> enabled. For > > >> # example, the guest uses ACPI to sleep, and ACPI sleep > > >> state > > >> # goes in real-mode > > >> # 3. Currently only supported on i386 and x86_64. > > >> # > > > > > > "virsh dump --memory-only" sets paging=false, for obvious reasons. > > > > > > [...] the dump-guest-memory command provides a raw snapshot of the > > > virtual machine's memory (and of the registers of the VCPUs); it is > > > not enlightened about the guest. > > > > > > If the additional information you are looking for can be retrieved > > > within the running Linux guest, using an appropriately privieleged > > > userspace process, then I would recommend considering an extension to > > > the qemu guest agent. The management layer (libvirt, [...]) could > > > first invoke the guest agent (a process with root privileges running > > > in the guest) from the host side, through virtio-serial. The new guest > > > agent command would return the information necessary to deal with > > > KASLR. Then the management layer would initiate the dump like always. > > > Finally, the extra information would be combined with (or placed > > > beside) the dump file in some way. > > > > > > So, this proposal would affect the guest agent and the management > > > layer (= libvirt). > > > > Given that we already dislike "paging=true", enlightening > > dump-guest-memory with even more guest-specific insight is the wrong > > approach, IMO. That kind of knowledge belongs to the guest agent. > > If you're trying to debug a hung/panicked guest, then using a guest > agent to fetch info is a complete non-starter as it'll be dead.
So don't wait. Management software can make this query immediately after the guest agent goes live. The information needed won't change. drew
