Re: [PATCH v12 05/10] arm64: Kprobes with single stepping support

[Masami Hiramatsu <mhiramat@kernel.org>]

On Tue, 17 May 2016 16:58:09 +0800
Huang Shijie <shijie.huang@arm.com> wrote:

> On Wed, Apr 27, 2016 at 02:53:00PM -0400, David Long wrote:
> > +
> > +/*
> > + * Interrupts need to be disabled before single-step mode is set, and not
> > + * reenabled until after single-step mode ends.
> > + * Without disabling interrupt on local CPU, there is a chance of
> > + * interrupt occurrence in the period of exception return and  start of
> > + * out-of-line single-step, that result in wrongly single stepping
> > + * into the interrupt handler.
> > + */
> > +static void __kprobes kprobes_save_local_irqflag(struct pt_regs *regs)
> > +{
> > +     struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> 
> Why not add a parameter for this function to save the @kcb?

Good catch, it should use same kcb of caller.

> 
> > +
> > +     kcb->saved_irqflag = regs->pstate;
> > +     regs->pstate |= PSR_I_BIT;
> > +}
> > +
> > +static void __kprobes kprobes_restore_local_irqflag(struct pt_regs *regs)
> > +{
> > +     struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> ditto.
> 
> > +
> > +     if (kcb->saved_irqflag & PSR_I_BIT)
> > +             regs->pstate |= PSR_I_BIT;
> > +     else
> > +             regs->pstate &= ~PSR_I_BIT;
> > +}
> > +
> > +static void __kprobes
> > +set_ss_context(struct kprobe_ctlblk *kcb, unsigned long addr)
> > +{
> > +     kcb->ss_ctx.ss_pending = true;
> > +     kcb->ss_ctx.match_addr = addr + sizeof(kprobe_opcode_t);
> > +}
> > +
> > +static void __kprobes clear_ss_context(struct kprobe_ctlblk *kcb)
> > +{
> > +     kcb->ss_ctx.ss_pending = false;
> > +     kcb->ss_ctx.match_addr = 0;
> > +}
> > +
> > +static void __kprobes setup_singlestep(struct kprobe *p,
> > +                                    struct pt_regs *regs,
> > +                                    struct kprobe_ctlblk *kcb, int reenter)
> > +{
> > +     unsigned long slot;
> > +
> > +     if (reenter) {
> > +             save_previous_kprobe(kcb);
> > +             set_current_kprobe(p);
> > +             kcb->kprobe_status = KPROBE_REENTER;
> > +     } else {
> > +             kcb->kprobe_status = KPROBE_HIT_SS;
> > +     }
> > +
> > +     if (p->ainsn.insn) {
> > +             /* prepare for single stepping */
> > +             slot = (unsigned long)p->ainsn.insn;
> > +
> > +             set_ss_context(kcb, slot);      /* mark pending ss */
> > +
> > +             if (kcb->kprobe_status == KPROBE_REENTER)
> > +                     spsr_set_debug_flag(regs, 0);
> > +
> > +             /* IRQs and single stepping do not mix well. */
> > +             kprobes_save_local_irqflag(regs);
> > +             kernel_enable_single_step(regs);
> > +             instruction_pointer(regs) = slot;
> > +     } else  {
> > +             BUG();

You'd better use BUG_ON(!p->ainsn.insn);

> > +     }
> > +}
> > +
> > +static int __kprobes reenter_kprobe(struct kprobe *p,
> > +                                 struct pt_regs *regs,
> > +                                 struct kprobe_ctlblk *kcb)
> > +{
> > +     switch (kcb->kprobe_status) {
> > +     case KPROBE_HIT_SSDONE:
> > +     case KPROBE_HIT_ACTIVE:
> > +             kprobes_inc_nmissed_count(p);
> > +             setup_singlestep(p, regs, kcb, 1);
> > +             break;
> > +     case KPROBE_HIT_SS:
> > +     case KPROBE_REENTER:
> > +             pr_warn("Unrecoverable kprobe detected at %p.\n", p->addr);
> > +             dump_kprobe(p);
> > +             BUG();
> > +             break;
> > +     default:
> > +             WARN_ON(1);
> > +             return 0;
> > +     }
> > +
> > +     return 1;
> > +}
> > +
> > +static void __kprobes
> > +post_kprobe_handler(struct kprobe_ctlblk *kcb, struct pt_regs *regs)
> > +{
> > +     struct kprobe *cur = kprobe_running();
> > +
> > +     if (!cur)
> > +             return;
> > +
> > +     /* return addr restore if non-branching insn */
> > +     if (cur->ainsn.restore.type == RESTORE_PC) {
> > +             instruction_pointer(regs) = cur->ainsn.restore.addr;
> > +             if (!instruction_pointer(regs))
> > +                     BUG();
> > +     }
> > +
> > +     /* restore back original saved kprobe variables and continue */
> > +     if (kcb->kprobe_status == KPROBE_REENTER) {
> > +             restore_previous_kprobe(kcb);
> > +             return;
> > +     }
> > +     /* call post handler */
> > +     kcb->kprobe_status = KPROBE_HIT_SSDONE;
> > +     if (cur->post_handler)  {
> > +             /* post_handler can hit breakpoint and single step
> > +              * again, so we enable D-flag for recursive exception.
> > +              */
> > +             cur->post_handler(cur, regs, 0);
> > +     }
> > +
> > +     reset_current_kprobe();
> > +}
> > +
> > +int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
> > +{
> > +     struct kprobe *cur = kprobe_running();
> > +     struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> > +
> > +     switch (kcb->kprobe_status) {
> > +     case KPROBE_HIT_SS:
> > +     case KPROBE_REENTER:
> > +             /*
> > +              * We are here because the instruction being single
> > +              * stepped caused a page fault. We reset the current
> > +              * kprobe and the ip points back to the probe address
> > +              * and allow the page fault handler to continue as a
> > +              * normal page fault.
> > +              */
> > +             instruction_pointer(regs) = (unsigned long)cur->addr;
> > +             if (!instruction_pointer(regs))
> > +                     BUG();

This can be BUG_ON(!instruction_pointer(regs)).

> > +             if (kcb->kprobe_status == KPROBE_REENTER)
> > +                     restore_previous_kprobe(kcb);
> > +             else
> > +                     reset_current_kprobe();
> > +
> > +             break;
> > +     case KPROBE_HIT_ACTIVE:
> > +     case KPROBE_HIT_SSDONE:
> > +             /*
> > +              * We increment the nmissed count for accounting,
> > +              * we can also use npre/npostfault count for accounting
> > +              * these specific fault cases.
> > +              */
> > +             kprobes_inc_nmissed_count(cur);
> > +
> > +             /*
> > +              * We come here because instructions in the pre/post
> > +              * handler caused the page_fault, this could happen
> > +              * if handler tries to access user space by
> > +              * copy_from_user(), get_user() etc. Let the
> > +              * user-specified handler try to fix it first.
> > +              */
> > +             if (cur->fault_handler && cur->fault_handler(cur, regs, fsr))
> > +                     return 1;
> > +
> > +             /*
> > +              * In case the user-specified fault handler returned
> > +              * zero, try to fix up.
> > +              */
> > +             if (fixup_exception(regs))
> > +                     return 1;
> > +     }
> > +     return 0;
> > +}
> > +
> > +int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
> > +                                    unsigned long val, void *data)
> > +{
> > +     return NOTIFY_DONE;
> > +}
> > +
> > +static void __kprobes kprobe_handler(struct pt_regs *regs)
> > +{
> > +     struct kprobe *p, *cur_kprobe;
> > +     struct kprobe_ctlblk *kcb;
> > +     unsigned long addr = instruction_pointer(regs);
> > +
> > +     kcb = get_kprobe_ctlblk();
> > +     cur_kprobe = kprobe_running();
> > +
> > +     p = get_kprobe((kprobe_opcode_t *) addr);
> > +
> > +     if (p) {
> > +             if (cur_kprobe) {
> > +                     if (reenter_kprobe(p, regs, kcb))
> > +                             return;
> > +             } else {
> > +                     /* Probe hit */
> > +                     set_current_kprobe(p);
> > +                     kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> > +
> > +                     /*
> > +                      * If we have no pre-handler or it returned 0, we
> > +                      * continue with normal processing.  If we have a
> > +                      * pre-handler and it returned non-zero, it prepped
> > +                      * for calling the break_handler below on re-entry,
> > +                      * so get out doing nothing more here.
> > +                      *
> > +                      * pre_handler can hit a breakpoint and can step thru
> > +                      * before return, keep PSTATE D-flag enabled until
> > +                      * pre_handler return back.
> > +                      */
> > +                     if (!p->pre_handler || !p->pre_handler(p, regs)) {
> > +                             kcb->kprobe_status = KPROBE_HIT_SS;
> The above line is duplicated.
> You will set KPROBE_HIT_SS in the setup_singlestep.

Right.

> 
> > +                             setup_singlestep(p, regs, kcb, 0);
> > +                             return;
> > +                     }
> > +             }
> > +     } else if ((le32_to_cpu(*(kprobe_opcode_t *) addr) ==
> > +         BRK64_OPCODE_KPROBES) && cur_kprobe) {
> > +             /* We probably hit a jprobe.  Call its break handler. */
> > +             if (cur_kprobe->break_handler  &&
> > +                  cur_kprobe->break_handler(cur_kprobe, regs)) {
> > +                     kcb->kprobe_status = KPROBE_HIT_SS;
> ditto
> > +                     setup_singlestep(cur_kprobe, regs, kcb, 0);
> > +                     return;
> > +             }
> > +     }
> > +     /*
> > +      * The breakpoint instruction was removed right
> > +      * after we hit it.  Another cpu has removed
> > +      * either a probepoint or a debugger breakpoint
> > +      * at this address.  In either case, no further
> > +      * handling of this interrupt is appropriate.
> > +      * Return back to original instruction, and continue.
> > +      */
> > +}

Thanks,



-- 
Masami Hiramatsu <mhiramat@kernel.org>