Re: [GIT PULL] x86/kaslr for v3.14

From: Vivek Goyal
Date: Fri Feb 07 2014 - 11:25:23 EST

On Fri, Feb 07, 2014 at 08:04:13AM -0800, H. Peter Anvin wrote:
> On 02/07/2014 06:49 AM, Vivek Goyal wrote:
> >
> > Hi Kees,
> >
> > Dave Young is testing kdump with kaslr enabled. He is facing some issues.
> >
> > One issue he mentioned is that when second kernel boots, it might be
> > placed in an area which is outside the reserved area for second kernel.
> >
> > We reserve a certain memory for second kernel. And modify memory map of
> > second kernel using memmap=exactmap parameter. Looks like kernel placement
> > is happening before memmap=exactmap takes effect. And that seems to be
> > the reason that second kernel can be placed outside the reserved memory.
> >
> > IOW, memmap=exactmap and kaslr don't work together. Is it possible to
> > first let memmap=exactmap take affect and then kaslr does its job. Or it
> > is too late by the time memmap=exactmap is parsed.
> >
> > As a workaround, Dave is currently using "nokaslr" command line parameter
> > for second kernel. He is still facing issues where makedumpfile segment
> > faults. He is looking into it further.
> >
> > I thought I will atleast bring up with issue of memmap=exactmap and kaslr
> > being incompatible.
> >
> Yes, because memmap=exactmap gets parsed too late; kaslr assumes that
> the e820 information passed to it is actually correct.
> Yet another cause of breakage caused by the decision on the part of
> kdump to rely on command-line options.

[CC kexec mailing list]

Ok, I think this is high time we change kexec-tools to not use
memmap=exactmap and start passing modified memory map in bootparams. I
think only concern with that change was backward compatibility of
kexec-tools with older kernels.

IIUC, only thing which will be impacted by this change is users of
saved_max_pfn which determine the highest accessible pfn in first
kernel. Some calgary IOMMU code seems to be the only user of it now.

So may be we can create a new command line option say --pass-memmap-cmdline
to kexec-tools which forces old behavior and by default we pass memmap
in bootparams.

Or create --do-not-pass-memmap-cmdline and new users will use it. Default
will be old behavior.

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