Re: [PATCH] uprobes/x86: emulate push insns for uprobe on x86

[Yonghong Song]

On 11/9/17 5:44 AM, Oleg Nesterov wrote:
> On 11/09, Yonghong Song wrote:
> > This patch extends the emulation to "push <reg>"
> > insns. These insns are typical in the beginning
> > of the function. For example, bcc
> > in https://github.com/iovisor/bcc repo provides
> > tools to measure funclantency, detect memleak, etc.
> > The tools will place uprobes in the beginning of
> > function and possibly uretprobes at the end of function.
> > This patch is able to reduce the trap overhead for
> > uprobe from 2 to 1.
>
> OK. but to be honest I do not like the implementation, please see below.
>
> > +enum uprobe_insn_t {
> > +	UPROBE_BRANCH_INSN	= 0,
> > +	UPROBE_PUSH_INSN	= 1,
> > +};
> > +
> >   struct uprobe_xol_ops;
> >
> >   struct arch_uprobe {
> > @@ -42,6 +47,7 @@ struct arch_uprobe {
> >   	};
> >
> >   	const struct uprobe_xol_ops	*ops;
> > +	enum uprobe_insn_t		insn_class;
>
> Why?
>
> I'd suggest to leave branch_xol_ops alone and add the new push_xol_ops{},
> the code will look much simpler.
>
> The only thing they can share is branch_post_xol_op() which is just
>
> 	regs->sp += sizeof_long();
> 	return -ERESTART;
>
> I think a bit of code duplication would be fine in this case.

Just prototyped. Agreed, having seperate uprobe_xol_ops for "push" 
emulation is clean and better.

> And. Do you really need ->post_xol() method to emulate "push"? Why we can't
> simply execute it out-of-line if copy_to_user() fails?

Thanks for pointing it out. Agreed, we do not really need post_xol for 
"push". xol execution is just fine.

Will address your other comments as well in the next revision.

> branch_post_xol_op() is needed because we can't execute "call" out-of-line,
> we need to restart and try again if copy_to_user() fails, but I don not
> understand why it is needed to emulate "push".
>
> Oleg.