Date: Thu, 23 Apr 2015 22:58:13 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: mingo@kernel.org, jpoimboe@redhat.com, sjenning@redhat.com,
        jkosina@suse.cz, vojtech@suse.cz, catalin.marinas@arm.com,
        will.deacon@arm.com, broonie@kernel.org,
        masami.hiramatsu.pt@hitachi.com, live-patching@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linaro-kernel@lists.linaro.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC 0/4] arm64: add livepatch support
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On Fri, 24 Apr 2015 11:44:05 +0900
AKASHI Takahiro <takahiro.akashi@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@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
> 

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