On Sat, 5 May 2018 00:48:28 +0900 Masami Hiramatsu mhiramat@kernel.org wrote:
So the syntax will be
p[:EVENT] SYM[(CAST)|+OFFS] [FETCHARG]
And here is an example;
p:myevent vfs_read(void *file, char *buf, size_t count, void *pos) $arg1 $arg2
If we do this, why bother with $arg1 $arg2?
We could allow this to be an alternative format?
In this case inside '()' will be analyzed and packed as something like "reference type" data and it is used when converting "$argN". And maybe we can provide $args special variable to record all arguments (it can be available only when the (CAST) is given).
This gives the user a consistent model; if you just give a symbol the arguments may not be correctly translated. but if you give a type-casting information, it will be much better.
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 */
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.
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