From: mhiramat@kernel.org (Masami Hiramatsu)
Subject: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features
Date: Mon, 7 May 2018 23:53:17 +0900 [thread overview]
Message-ID: <20180507235317.fa04131c603dbe59ef78a829@kernel.org> (raw)
Message-ID: <20180507145317.p0BBos4yYjRg1t4-g3Sqs7w8w0Qtcxj4BSY9JQAAajk@z> (raw)
In-Reply-To: <1525680222.8nou0tzkkt.naveen@linux.ibm.com>
On Mon, 07 May 2018 13:41:53 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> Masami Hiramatsu wrote:
> > On Sat, 05 May 2018 13:16:04 +0530
> > "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
> >
> >> Masami Hiramatsu wrote:
> >> > On Fri, 4 May 2018 12:06:42 -0400
> >> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >> >
> >> >> On Sat, 5 May 2018 00:48:28 +0900
> >> >> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >> >>
> >> >> > > Also, when looking at the kprobe code, I was looking at this
> >> >> > > function:
> >> >> > >
> >> >> > > > /* 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;
> >> >> > > >
> >> >> > > > /* Preempt is disabled by ftrace */
> >> >> > > > 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 {
> >> >> > > > unsigned long orig_ip = regs->ip;
> >> >> > > > /* Kprobe handler expects regs->ip = ip + 1 as breakpoint hit */
> >> >> > > > regs->ip = ip + sizeof(kprobe_opcode_t);
> >> >> > > >
> >> >> > > > /* To emulate trap based kprobes, preempt_disable here */
> >> >> > > > preempt_disable();
> >> >> > > > __this_cpu_write(current_kprobe, p);
> >> >> > > > kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> >> >> > > > if (!p->pre_handler || !p->pre_handler(p, regs)) {
> >> >> > > > __skip_singlestep(p, regs, kcb, orig_ip);
> >> >> > > > preempt_enable_no_resched();
> >> >> > >
> >> >> > > This preemption disabling and enabling looks rather strange. Looking at
> >> >> > > git blame, it appears this was added for jprobes. Can we remove it now
> >> >> > > that jprobes is going away?
> >> >> >
> >> >> > No, that is not for jprobes but for compatibility with kprobe's user
> >> >> > handler. Since this transformation is done silently, user can not
> >> >> > change their handler for ftrace case. So we need to keep this condition
> >> >> > same as original kprobes.
> >> >> >
> >> >> > And anyway, for using smp_processor_id() for accessing per-cpu,
> >> >> > we should disable preemption, correct?
> >> >>
> >> >> But as stated at the start of the function:
> >> >>
> >> >> /* Preempt is disabled by ftrace */
> >> >
> >> > Ah, yes. So this is only for the jprobes.
> >> >
> >> >>
> >> >>
> >> >> The reason I ask, is that we have for this function:
> >> >>
> >> >> /* To emulate trap based kprobes, preempt_disable here */
> >> >> preempt_disable();
> >> >> __this_cpu_write(current_kprobe, p);
> >> >> kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> >> >> if (!p->pre_handler || !p->pre_handler(p, regs)) {
> >> >> __skip_singlestep(p, regs, kcb, orig_ip);
> >> >> preempt_enable_no_resched();
> >> >> }
> >> >>
> >> >> And in arch/x86/kernel/kprobes/core.c we have:
> >> >>
> >> >> preempt_disable();
> >> >>
> >> >> kcb = get_kprobe_ctlblk();
> >> >> p = get_kprobe(addr);
> >> >>
> >> >> if (p) {
> >> >> if (kprobe_running()) {
> >> >> if (reenter_kprobe(p, regs, kcb))
> >> >> return 1;
> >> >> } else {
> >> >> set_current_kprobe(p, regs, kcb);
> >> >> 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
> >> >> * for jprobe processing, so get out doing nothing
> >> >> * more here.
> >> >> */
> >> >> if (!p->pre_handler || !p->pre_handler(p, regs))
> >> >> setup_singlestep(p, regs, kcb, 0);
> >> >> return 1;
> >> >>
> >> >>
> >> >> Which is why I thought it was for jprobes. I'm a bit confused about
> >> >> where preemption is enabled again.
> >> >
> >> > You're right. So I would like to remove it with x86 jprobe support
> >> > code to avoid inconsistency.
> >>
> >> I didn't understand that. Which code are you planning to remove? Can you
> >> please elaborate? I thought we still need to disable preemption in the
> >> ftrace handler.
> >
> > Yes, kprobe_ftrace_handler itself must be run under preempt disabled
> > because it depends on a per-cpu variable. What I will remove is the
> > redundant preempt disable/enable_noresched (unbalanced) pair in the
> > kprobe_ftrace_handler, and jprobe x86 ports which is no more used.
>
> Won't that break out-of-tree users depending on returning a non-zero
> value to handle preemption differently? You seem to have alluded to it
> earlier in the mail chain above where you said that this is not just for
> jprobes (though it was added for jprobes as the main use case).
No, all users are in tree already (function override for bpf and error-injection).
And also, for changing execution path by using kprobes, user handler must call
not only preempt_enable(), but also clear current_kprobe per-cpu variable which
is not exported to kmodules.
This means if there is such out-of-tree module, that must change the kernel or
hack the kernel to identify the address of curent_kprobe. If it requires such
a change or hack for the kernel, it is very easy to update the module too.
Thank you,
>
> - Naveen
>
>
--
Masami Hiramatsu <mhiramat at kernel.org>
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next prev parent reply other threads:[~2018-05-07 14:53 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-25 12:16 [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features mhiramat
2018-04-25 12:16 ` Masami Hiramatsu
2018-04-25 12:16 ` [PATCH v7 01/16] tracing: probeevent: Cleanup print argument functions mhiramat
2018-04-25 12:16 ` Masami Hiramatsu
2018-04-25 12:17 ` [PATCH v7 02/16] tracing: probeevent: Cleanup argument field definition mhiramat
2018-04-25 12:17 ` Masami Hiramatsu
2018-04-25 12:17 ` [PATCH v7 03/16] tracing: probeevent: Remove NOKPROBE_SYMBOL from print functions mhiramat
2018-04-25 12:17 ` Masami Hiramatsu
2018-04-25 12:18 ` [PATCH v7 04/16] tracing: probeevent: Introduce new argument fetching code mhiramat
2018-04-25 12:18 ` Masami Hiramatsu
2018-04-25 12:18 ` [PATCH v7 05/16] tracing: probeevent: Unify fetch type tables mhiramat
2018-04-25 12:18 ` Masami Hiramatsu
2018-04-25 12:19 ` [PATCH v7 06/16] tracing: probeevent: Return consumed bytes of dynamic area mhiramat
2018-04-25 12:19 ` Masami Hiramatsu
2018-04-25 12:19 ` [PATCH v7 07/16] tracing: probeevent: Append traceprobe_ for exported function mhiramat
2018-04-25 12:19 ` Masami Hiramatsu
2018-04-25 12:19 ` [PATCH v7 08/16] tracing: probeevent: Unify fetch_insn processing common part mhiramat
2018-04-25 12:19 ` Masami Hiramatsu
2018-04-25 12:20 ` [PATCH v7 09/16] tracing: probeevent: Add symbol type mhiramat
2018-04-25 12:20 ` Masami Hiramatsu
2018-04-25 12:20 ` [PATCH v7 10/16] x86: ptrace: Add function argument access API mhiramat
2018-04-25 12:20 ` Masami Hiramatsu
2018-04-25 12:21 ` [PATCH v7 11/16] tracing: probeevent: Add $argN for accessing function args mhiramat
2018-04-25 12:21 ` Masami Hiramatsu
2018-04-25 12:21 ` [PATCH v7 12/16] tracing: probeevent: Add array type support mhiramat
2018-04-25 12:21 ` Masami Hiramatsu
2018-04-25 12:22 ` [PATCH v7 13/16] selftests: ftrace: Add a testcase for symbol type mhiramat
2018-04-25 12:22 ` Masami Hiramatsu
2018-04-25 12:22 ` [PATCH v7 14/16] selftests: ftrace: Add a testcase for $argN with kprobe_event mhiramat
2018-04-25 12:22 ` Masami Hiramatsu
2018-04-25 12:23 ` [PATCH v7 15/16] selftests: ftrace: Add a testcase for array type " mhiramat
2018-04-25 12:23 ` Masami Hiramatsu
2018-04-25 12:23 ` [PATCH v7 16/16] perf-probe: Add array argument support mhiramat
2018-04-25 12:23 ` Masami Hiramatsu
2018-04-27 1:42 ` [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features rostedt
2018-04-27 1:42 ` Steven Rostedt
2018-05-03 22:11 ` rostedt
2018-05-03 22:11 ` Steven Rostedt
2018-05-04 15:48 ` mhiramat
2018-05-04 15:48 ` Masami Hiramatsu
2018-05-04 16:06 ` rostedt
2018-05-04 16:06 ` Steven Rostedt
2018-05-04 17:30 ` naveen.n.rao
2018-05-04 17:30 ` Naveen N. Rao
2018-05-05 2:38 ` mhiramat
2018-05-05 2:38 ` Masami Hiramatsu
2018-05-05 7:46 ` naveen.n.rao
2018-05-05 7:46 ` Naveen N. Rao
2018-05-05 14:32 ` mhiramat
2018-05-05 14:32 ` Masami Hiramatsu
2018-05-07 8:11 ` naveen.n.rao
2018-05-07 8:11 ` Naveen N. Rao
2018-05-07 14:53 ` mhiramat [this message]
2018-05-07 14:53 ` Masami Hiramatsu
2018-05-08 10:11 ` naveen.n.rao
2018-05-08 10:11 ` Naveen N. Rao
2018-05-08 15:02 ` mhiramat
2018-05-08 15:02 ` Masami Hiramatsu
2018-05-08 18:01 ` naveen.n.rao
2018-05-08 18:01 ` Naveen N. Rao
2018-05-05 15:51 ` mhiramat
2018-05-05 15:51 ` Masami Hiramatsu
2018-05-07 15:30 ` rostedt
2018-05-07 15:30 ` Steven Rostedt
2018-05-08 4:01 ` mhiramat
2018-05-08 4:01 ` Masami Hiramatsu
2018-05-07 15:21 ` rostedt
2018-05-07 15:21 ` Steven Rostedt
2018-06-21 20:16 ` rostedt
2018-06-21 20:16 ` Steven Rostedt
2018-06-22 6:04 ` mhiramat
2018-06-22 6:04 ` Masami Hiramatsu
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