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

[Masami Hiramatsu]

On Mon, 8 Oct 2018 23:55:34 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> 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.

OK, the ideal solution sounds good to me. 

> 
> > 
> > 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.

Yeah, but we can remove the trampoline code.

> > 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.

OK, so we may add a note that this is "global" patching :)

> 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).

Indeed.

Thanks,

> 
> -- Steve


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>