Re: [Xen-devel] [PATCH v5 06/16] livepatch: ARM/x86: Check displacement of old_addr and new_addr

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Sep 23, 2016 at 11:37:27AM -0400, Konrad Rzeszutek Wilk wrote:
> On Fri, Sep 23, 2016 at 03:36:27PM +0100, Ross Lagerwall wrote:
> > On 09/21/2016 06:32 PM, Konrad Rzeszutek Wilk wrote:
> > > If the distance is too big we are in trouble - as our relocation
> > > distance can surely be clipped, or still have a valid width - but
> > > cause an overflow of distance.
> > > 
> > > On various architectures the maximum displacement for a unconditional
> > > branch/jump varies. ARM32 is +/- 32MB, ARM64 is +/- 128MB while x86
> > > for 32-bit relocations is +/- 2G.
> > > 
> > > Note: On x86 we could use the 64-bit jmpq instruction which
> > > would provide much bigger displacement to do a jump, but we would
> > > still have issues with the new function not being able to reach
> > > any of the old functions (as all the relocations would assume 32-bit
> > > displacement). And "furthermore would require an register or
> > > memory location to load/store the address to." (From Jan).
> > > 
> > > On ARM the conditional branch supports even a smaller displacement
> > > but fortunately we are not using that.
> > 
> > Wouldn't this be simpler as a BUILD_BUG_ON() in arch_livepatch_init()?
> > 
> > Something like BUILD_BUG_ON(((XEN_VIRT_END - NR_CPUS * PAGE_SIZE) -
> > XEN_VIRT_START) > ARCH_LIVEPATCH_RANGE)?
> > 
> > And BUILD_BUG_ON(((XEN_VIRT_END - NR_CPUS * PAGE_SIZE) - XEN_VIRT_START) >
> > ARCH_LIVEPATCH_RANGE)
> > 
> > And something similar for ARM...
> 
> What does that have to with the payload? The displacement calculations(checks)
> are done when we load the payload.

Ooh, you are thinking of just making sure that the displacement in virtual
addresses _should_ be OK.

And that BUILD_BUG_ON would certainly do it.

But my thinking was more of the payload either having some wacky relocations 
(say
having an initial addendum that along with the XEN_VIRT_START -> XEN_VIRT_END)
causes us to be way past the right place (especially with ARM32 where we only
have 32MB distance). And having this extra check makes me sleep better at night.

Adding the BUILD_BUG_ON as a futher check is fine, but as a different patch.

> > 
> > -- 
> > Ross Lagerwall

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