Re: [PATCH v2 6/6] riscv: Add KPROBES_ON_FTRACE supported

[Masami Hiramatsu]

On Sat, 11 Jul 2020 09:32:01 +0800
Guo Ren <guoren@xxxxxxxxxx> wrote:

> Thx Masami,
> 
> On Fri, Jul 10, 2020 at 9:50 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >
> > Hi Guo,
> >
> > On Thu,  9 Jul 2020 02:19:14 +0000
> > guoren@xxxxxxxxxx wrote:
> >
> > > +/* Ftrace callback handler for kprobes -- called under preepmt disabed */
> > > +void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> > > +                        struct ftrace_ops *ops, struct pt_regs *regs)
> > > +{
> > > +     struct kprobe *p;
> > > +     struct kprobe_ctlblk *kcb;
> > > +
> > > +     p = get_kprobe((kprobe_opcode_t *)ip);
> > > +     if (unlikely(!p) || kprobe_disabled(p))
> > > +             return;
> > > +
> > > +     kcb = get_kprobe_ctlblk();
> > > +     if (kprobe_running()) {
> > > +             kprobes_inc_nmissed_count(p);
> > > +     } else {
> > > +             /*
> > > +              * The regs->epc hasn't been saved by SAVE_ALL in mcount-dyn.S
> > > +              * So no need to resume it, just for kprobe handler.
> > > +              */
> > > +             instruction_pointer_set(regs, ip);
> > > +             __this_cpu_write(current_kprobe, p);
> > > +             kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> > > +             if (!p->pre_handler || !p->pre_handler(p, regs)) {
> > > +                     /*
> > > +                      * Emulate singlestep (and also recover regs->pc)
> > > +                      * as if there is a nop
> > > +                      */
> > > +                     instruction_pointer_set(regs,
> > > +                             (unsigned long)p->addr + MCOUNT_INSN_SIZE);
> > > +                     if (unlikely(p->post_handler)) {
> > > +                             kcb->kprobe_status = KPROBE_HIT_SSDONE;
> > > +                             p->post_handler(p, regs, 0);
> > > +                     }
> >
> > Hmm, don't you need restoring the previous instruction pointer here?
> look at  riscv mcount-dyn.S SAVE_ALL function, sp frame lay out like this:
> -----------------------
> | return address |
> -----------------------
> | frame pointer   |
> -----------------------
> | pt_regs x1-x31|
> -----------------------
> It's not a complete pt_regs for the handler, so modifing regs->ip is no use.

Yes, that is same on x86. But ftrace regs_caller on x86-64 modifies the
return address on the stack by regs->ip.

See arch/x86/kernel/ftrace_64.S:

-----
SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
        call ftrace_stub

        /* Copy flags back to SS, to restore them */
        movq EFLAGS(%rsp), %rax
        movq %rax, MCOUNT_REG_SIZE(%rsp)

        /* Handlers can change the RIP */
        movq RIP(%rsp), %rax
        movq %rax, MCOUNT_REG_SIZE+8(%rsp)
-----

The last part update the return address on the stack.

> > If you don't support modifying the instruction pointer in the handler,
> We can modify ip like this if necessary:
> *(unsigned long *)((unsigned long)regs + sizeof(struct pt_regs) + 8) = xxx;
> 
> > it must not be compatible with kprobes.
> Why, can you show related codes? thank you very much.

The "Changing Execution Path" section in the Documentation/kprobes.txt said

-----
Since kprobes can probe into a running kernel code, it can change the
register set, including instruction pointer.
-----

As you can see, this is the expected behavior on kprobes.
Since user will not know the kprobes is working on a breakpoint or
ftrace, we have to support this behavior transparently. So, could you
enable this feature at first on ftrace@RISCV?

> 
> >
> > Now BPF function override and function error injection depends on
> > this behevior, so could you consider to support it in the "ftrace"
> > implementation at first? (And if it is enabled, you can enable the
> > livepatch on RISCV too)
> Great message!
> 
> But can you show me codes that bpf and err-jnject using the behavior? Thx

In kernel/fail_function.c

-----
static int fei_kprobe_handler(struct kprobe *kp, struct pt_regs *regs)
{
        struct fei_attr *attr = container_of(kp, struct fei_attr, kp);

        if (should_fail(&fei_fault_attr, 1)) {
                regs_set_return_value(regs, attr->retval);
                override_function_with_return(regs);
                return 1;
        }

        return 0;
}
-----

And follow the code related to CONFIG_BPF_KPROBE_OVERRIDE in
kernel/trace/bpf_trace.c. (doing similar thing, but fail_function.c
is simpler.)

Thank you,


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>