Re: [PATCH 0/3] x86_64/ftrace: Emulate calls from int3 when patching functions

[Masami Hiramatsu]

On Tue, 07 May 2019 21:55:59 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> 
> [
>   This is the non-RFC version.
> 
>   It went through and passed all my tests. If there's no objections
>   I'm going to include this in my pull request. I still have patches
>   in my INBOX that may still be included, so I need to run those through
>   my tests as well, so a pull request wont be immediate.
> ]
> 
> Nicolai Stange discovered that Live Kernel Patching can have unforseen
> consequences if tracing is enabled when there are functions that are
> patched. The reason being, is that Live Kernel patching is built on top
> of ftrace, which will have the patched functions call the live kernel
> trampoline directly, and that trampoline will modify the regs->ip address
> to return to the patched function.
> 
> But in the transition between changing the call to the customized
> trampoline, the tracing code is needed to have its handler called
> an well, so the function fentry location must be changed from calling
> the live kernel patching trampoline, to the ftrace_reg_caller trampoline
> which will iterate through all the registered ftrace handlers for
> that function.
> 
> During this transition, a break point is added to do the live code
> modifications. But if that break point is hit, it just skips calling
> any handler, and makes the call site act as a nop. For tracing, the
> worse that can happen is that you miss a function being traced, but
> for live kernel patching the affects are more severe, as the old buggy
> function is now called.
> 
> To solve this, an int3_emulate_call() is created for x86_64 to allow
> ftrace on x86_64 to emulate the call to ftrace_regs_caller() which will
> make sure all the registered handlers to that function are still called.
> And this keeps live kernel patching happy!

Out of curiosity, would you have any idea to re-use these function for
other use-case? Maybe kprobes can reuse it, but very limited use-case.

> To mimimize the changes, and to avoid controversial patches, this
> only changes x86_64. Due to the way x86_32 implements the regs->sp
> the complexity of emulating calls on that platform is too much for
> stable patches, and live kernel patching does not support x86_32 anyway.

This series looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thanks!

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>