Re: [RFC][PATCH] Randomize kernel base address on boot

[Dan Rosenberg]

On Thu, 2011-05-26 at 16:01 -0400, Vivek Goyal wrote:
> On Tue, May 24, 2011 at 04:31:45PM -0400, Dan Rosenberg wrote:
> 
> [..]
> >  ==============================================================
> >  
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 880fcb6..999ea82 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -1548,8 +1548,8 @@ config PHYSICAL_START
> >  	  If kernel is a not relocatable (CONFIG_RELOCATABLE=n) then
> >  	  bzImage will decompress itself to above physical address and
> >  	  run from there. Otherwise, bzImage will run from the address where
> > -	  it has been loaded by the boot loader and will ignore above physical
> > -	  address.
> > +	  it has been loaded by the boot loader, using the above physical
> > +	  address as a lower bound.
> >  
> >  	  In normal kdump cases one does not have to set/change this option
> >  	  as now bzImage can be compiled as a completely relocatable image
> > @@ -1595,7 +1595,31 @@ config RELOCATABLE
> >  
> >  	  Note: If CONFIG_RELOCATABLE=y, then the kernel runs from the address
> >  	  it has been loaded at and the compile time physical address
> > -	  (CONFIG_PHYSICAL_START) is ignored.
> > +	  (CONFIG_PHYSICAL_START) is solely used as a lower bound.
> > +
> 
> This does not sound too good. Overloading the definition of PHYSICAL_START
> with minimum address. The very definition of relocatable kernel is that
> it should be able to run from the physical address it has been loaded
> at (subjected to alignment constraints).
> 
> So I don't think overloading CONFIG_PHYSICAL_START definition is a good
> idea. In fact there is no reason that why kdump kernels should not run
> and boot below 16MB. So limiting those kernels to not load and run
> below 16MB is does not sound like good option to me.
> 

I'm going to revisit this part of the patch and think of a better way to
do this.

> Also randomization of kernel load address at run time will probably have
> some issues with crashkernel=X@Y address syntax. So far user knew what
> address first kernel is booting from and user could speicy where to 
> reserve memory. Now it might happen that user specified some memory
> to reserve and kernel decided to occupy that space resulting in failed
> memory reservation for crash kernel.
> 

Ok, added to the list of things to figure out.  Thanks.

> Thanks
> Vivek


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