Re: [PATCH] disable kaslr in kdump kernel

[Dave Young]

On 02/11/14 at 08:16am, Vivek Goyal wrote:
> On Tue, Feb 11, 2014 at 06:08:38PM +0800, Dave Young wrote:
> > 
> > KASLR does not work in kdump kernel because it's too early that mem=exactmap
> > has not been parsed.
> > 
> > Since KASLR does not make much sense for kdump kernel thus let's disable it
> > for kdump kernel. To check if it is a kdump kernel I just check the cmdline
> > param elfcorehdr just like is_kdump_kernel.
> > 
> > Signed-off-by: Dave Young <dyoung@xxxxxxxxxx>
> 
> I don't think it is a good idea. I don't like hardcoding second kernel's
> behavior. I rather vary second kernel's behavior based on command line
> parameters or based on values passed in bootparams.
> 
> So I am more than happy to pass command line option "nokaslr" instead
> of hardcoding this in kernel.

That's also fine to me, but I think we'd better add it to documentation?

> 
> Thanks
> Vivek
> 
> > ---
> >  linux-2.6/arch/x86/boot/compressed/aslr.c |    5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > Index: dyoung/git/linux-2.6/arch/x86/boot/compressed/aslr.c
> > ===================================================================
> > --- linux-2.6/arch/x86/boot/compressed/aslr.c
> > +++ linux-2.6/arch/x86/boot/compressed/aslr.c
> > @@ -293,6 +293,11 @@ unsigned char *choose_kernel_location(un
> >  		goto out;
> >  	}
> >  
> > +	if (cmdline_find_option("elfcorehdr", NULL, 0) != -1) {
> > +		debug_putstr("KASLR disabled...\n");
> > +		goto out;
> > +	}
> > +
> >  	/* Record the various known unsafe memory ranges. */
> >  	mem_avoid_init((unsigned long)input, input_size,
> >  		       (unsigned long)output, output_size);
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