Re: [Xen-devel] [PATCH v7 3/5] livepatch: NOP if func->new_addr is zero.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.09.16 at 18:57, <konrad.wilk@xxxxxxxxxx> wrote:
> The NOP functionality will NOP any of the code at
> the 'old_addr' or at 'name' if the 'new_addr' is zero.
> The purpose of this is to NOP out calls, such as:
> 
>  e8 <4-bytes-offset>
> 
> (5 byte insn), or on ARM a 4 byte insn for branching.
> 
> We need the EIP of where we need to the NOP, and that can
> be provided via the `old_addr` or `name`.
> 
> If the `old_addr` is provided we will NOP 'new_size'
> amount of bytes at that location.
> 
> The amount is up to 31 instructions if desired (which is
> the size of the opaque member). If there is a need to NOP
> more then: a) more 'struct livepatch_func' structures need
> to be present, b) we have to implement a variable size
> buffer (in the future), or c) first byte an unconditional
> branch skipping the to be disabled code (of course provided
> there are no branch targets in the middle).
> 
> While at it, also unify the code on x86 patching so
> it is a bit simpler (instead of two seperate writes
> just make it one memcpy).
> 
> And introduce a general livepatch_insn_len inline function
> that would depend on platform specific instruction size
> (for a unconditional branch). As such we also rename the
> PATCH_INSN_SIZE to ARCH_PATCH_INSN_SIZE.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>


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