Re: [PATCH 6/6] powerpc/livepatch: Add live patching support on ppc64le

From: Balbir Singh
Date: Thu Mar 31 2016 - 21:11:14 EST

On 24/03/16 22:04, Michael Ellerman wrote:
> Add the kconfig logic & assembly support for handling live patched
> functions. This depends on DYNAMIC_FTRACE_WITH_REGS, which in turn
> depends on the new -mprofile-kernel ftrace ABI, which is only supported
> currently on ppc64le.
> Live patching is handled by a special ftrace handler. This means it runs
> from ftrace_caller(). The live patch handler modifies the NIP so as to
> redirect the return from ftrace_caller() to the new patched function.
> However there is one particularly tricky case we need to handle.
> If a function A calls another function B, and it is known at link time
> that they share the same TOC, then A will not save or restore its TOC,
> and will call the local entry point of B.
> When we live patch B, we replace it with a new function C, which may
> not have the same TOC as A. At live patch time it's too late to modify A
> to do the TOC save/restore, so the live patching code must interpose
> itself between A and C, and do the TOC save/restore that A omitted.
> An additionaly complication is that the livepatch code can not create a
> stack frame in order to save the TOC. That is because if C takes > 8
> arguments, or is varargs, A will have written the arguments for C in
> A's stack frame.
> To solve this, we introduce a "livepatch stack" which grows upward from
> the base of the regular stack, and is used to store the TOC & LR when
> calling a live patched function.
> When the patched function returns, we retrieve the real LR & TOC from
> the livepatch stack, restore them, and pop the livepatch "stack frame".
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

Reviewed-by: Balbir Singh <bsingharora@xxxxxxxxx>