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 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.
> 
> -- 
> Ross Lagerwall

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