Re: [RFC 0/4] arm64: add livepatch support

[Li Bin <huawei.libin@huawei.com>]

On 2015/4/24 14:05, Masami Hiramatsu wrote:
> (2015/04/24 12:24), Li Bin wrote:
>> On 2015/4/24 10:44, AKASHI Takahiro wrote:
>>> This patchset enables livepatch support on arm64.
>>>
>>> Livepatch was merged in v4.0, and allows replacying a function dynamically
>>> based on ftrace framework, but it also requires -mfentry option of gcc.
>>> Currently arm64 gcc doesn't support it, but by adding a helper function to
>>> ftrace, we will be able to support livepatch on arch's which don't support
>>> this option.
>>>
>>
>> This is not correct for the case that the prologue of the old and new function
>> is different.
> 
> Hmm, is that possible to support -mfentry on arm64?
> 
> Of course we can not call a function directly at the first
> instruction of functions on arm, because it can overwrite
> link register which stores caller address. However, we can
> do "store link register to stack and branch with link"
> on arm. That is actually almost same as -mfentry does :),
> and that may not depend on the prologue.
>

Yes, but the method "store link register to stack" will break the arm64 ABI rules,
and memory operations may bring performance cost.
We have a gcc -mfentry implementation strategy for arm64, and based on this we
implement the livepatch for arm64.
I will post the patchset for review soon.
Thank you,
	Li Bin

> Thank you,
> 
> 
>> Thanks,
>> 	Li Bin
>>
>>> I submit this patchset as RFC since I'm not quite sure that I'm doing
>>> in the right way, or we should definitely support -fentry instead.
>>>
>>> Please note that I tested the feature only with livepatch-sample, and
>>> the code for DYNAMIC_TRACE_WITH_REGS is still rough-edged.
>>>
>>> To: Steven Rostedt <rostedt@goodmis.org>
>>> To: Ingo Molnar <mingo@kernel.org>
>>> To: Josh Poimboeuf <jpoimboe@redhat.com>
>>> To: Seth Jennings <sjenning@redhat.com>
>>> To: Jiri Kosina <jkosina@suse.cz>
>>> To: Vojtech Pavlik <vojtech@suse.cz>
>>> To: Catalin Marinas <catalin.marinas@arm.com>
>>> To: Will Deacon <will.deacon@arm.com>
>>>
>>> AKASHI Takahiro (4):
>>>   ftrace: add a helper function for livepatch
>>>   livepatch: adjust a patched function's address
>>>   arm64: ftrace: add DYNAMIC_TRACE_WITH_REGS version
>>>   arm64: add livepatch support
>>>
>>>  arch/arm64/Kconfig                 |    4 ++
>>>  arch/arm64/include/asm/ftrace.h    |    4 ++
>>>  arch/arm64/include/asm/livepatch.h |   38 +++++++++++
>>>  arch/arm64/kernel/Makefile         |    1 +
>>>  arch/arm64/kernel/entry-ftrace.S   |  124 ++++++++++++++++++++++++++++++++++++
>>>  arch/arm64/kernel/ftrace.c         |   24 ++++++-
>>>  arch/arm64/kernel/livepatch.c      |   68 ++++++++++++++++++++
>>>  arch/x86/include/asm/livepatch.h   |    5 ++
>>>  include/linux/ftrace.h             |    2 +
>>>  include/linux/livepatch.h          |    2 +
>>>  kernel/livepatch/core.c            |   16 +++--
>>>  kernel/trace/ftrace.c              |   26 ++++++++
>>>  12 files changed, 309 insertions(+), 5 deletions(-)
>>>  create mode 100644 arch/arm64/include/asm/livepatch.h
>>>  create mode 100644 arch/arm64/kernel/livepatch.c
>>>
>>
>>
>>
> 
>