[RFC 0/4] arm64: add livepatch support

[Steven Rostedt]

On Fri, 24 Apr 2015 11:44:05 +0900
AKASHI Takahiro <takahiro.akashi at linaro.org> 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.
> 
> 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.

You need to be extremely careful about this. I don't know what arm does
with mcount but on x86 without -fentry, it sets up the stack frame
before calling mcount. That is, if mcount returns to a different
function, it doesn't mean that the registers will match the parameters.

I have to look at what arm64 does when compiled with -pg. Because, if
it moves registers around before the mcount, you will have a disaster
on your hands if it returns to a different function that moved the
registers around differently.

Also, you need to be careful about how link registers are handled.

-- Steve


> 
> 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 at goodmis.org>
> To: Ingo Molnar <mingo at kernel.org>
> To: Josh Poimboeuf <jpoimboe at redhat.com>
> To: Seth Jennings <sjenning at redhat.com>
> To: Jiri Kosina <jkosina at suse.cz>
> To: Vojtech Pavlik <vojtech at suse.cz>
> To: Catalin Marinas <catalin.marinas at arm.com>
> To: Will Deacon <will.deacon at 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
>