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

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

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...

--
Ross Lagerwall

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