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

[Masami Hiramatsu]

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 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)

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?).


Thank you,


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>