Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"

[Steven Rostedt]

On Tue, 9 Oct 2018 12:44:01 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> On Fri, 05 Oct 2018 21:51:11 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > +typedef long dynfunc_t;
> > +
> > +struct dynfunc_struct;
> > +
> > +#define arch_dynfunc_trampoline(name, def)	\
> > +	asm volatile (				\
> > +	".globl dynfunc_" #name "; \n\t"	\
> > +	"dynfunc_" #name ": \n\t"		\
> > +	"jmp " #def " \n\t"			\
> > +	".balign 8 \n \t"			\
> > +	: : : "memory" )
> > +  
> 
> I have just a question, what is this different from livepatch? :)

I actually thought about this a bit, but decided against it.

I didn't want to hook another infrastructure into the fentry nop. It's
already complex enough with kprobes, live patching and ftrace.

The ideal solution is what Peter suggested, and that's to patch the
call sites, and I think that is attainable with objtool modifications.

> 
> I think we can replace the first 5 bytes of the default function
> to jmp instruction (to alternative function) instead of making
> this trampoline.
> 
> IOW, as far as I can see, this is changing
> 
> ----
> call %reg (or retpoline_reg)
> ----
> 
> to 
> 
> ----
> call dynfunc_A
> 
> dynfunc_A:
> jmp func_A or altered_func_A
> ----
> 
> If so, why don't we put the jmp on default func_A directly?
> ----
> call func_A
> 
> func_A:
> "jmp altered_func" or "original sequence"
> ----
> (this is idealy same as jprobes did)
> 
> Of course we have to arbitrate it with ftrace (fentry) but it may
> not so hard (simplest way is just adding "notrace" on the default
> function)

Then we lose the 5 byte nop.

> 
> BTW, I think "dynamic_function" may not correct name, it may be
> "alternative_function" or something like that, because this
> function must be replaced system-wide and this means we can
> not use this for generic function pointer usage which depends
> on thread context (like file_operations). But good for something
> pluggable code (LSM?).

I don't like the name alternative, as that's usually a one shot deal
(SMP vs UP).

It is dynamic, as it's a function that changes dynamically. Yes its
global, but that's not mutually exclusive to dynamic.

The use case I want this for is for tracing. But it can be useful for
KVM and power management governors. Basically anything that has a
global function pointer (hmm, even the idle call can use this).

-- Steve