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SubjectRe: [PATCH v7 00/16] tracing: probeevent: Improve fetcharg features
On Wed, 25 Apr 2018 21:16:06 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Hi,
>
> This is the 7th version of the fetch-arg improvement series.
> This includes variable changes on fetcharg framework like,
>
> - Add fetcharg testcases (syntax, argN, symbol, string and array)
> and probepoint testcase.
> - Rewrite fetcharg framework with fetch_insn, switch-case based
> instead of function pointer.
> - Add "symbol" type support, which shows symbol+offset instead of
> address value.
> - Add "$argN" fetcharg, which fetches function parameters.
> (currently only for x86-64)
> - Add array type support (including string arrary :) ) ,
> which enables to get fixed length array from probe-events.
> - Add array type support for perf-probe, so that user can
> dump partial array entries.
>
> V6 is here:
> https://lkml.org/lkml/2018/3/17/75
>
> Changes from the v6 are here:
> [6/16] - Fix to return an error if failed to fetch string and
> fill zero-length data_loc in error case.
> [11/16] - Update document for restructured text.
> [15/16] - Fix README test.
> [16/16] - Add type-casting description (and note) to documentation.
>
> And rebased on the latest Steve's ftrace/core branch.
>

Hi Masami,

I skimmed through the patches and it they appear fine. I've applied
them and started playing a little.

I've been thinking my function based events, and thought instead I
would make them part of the kprobe infrastructure. Just have a slightly
different format, and instead of being p: or r: be f: And keep the
format I was suggesting.

What do you think?

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?

> }
> /*
> * If pre_handler returns !0, it sets regs->ip and
> * resets current kprobe, and keep preempt count +1.
> */
> }
> }

-- Steve

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