Re: [PATCH][RFC] tracing: Enable tracepoints via module parameters

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On 20/04/2021 22.32, Steven Rostedt wrote:
> On Tue, 20 Apr 2021 12:54:39 -0700
> Dan Williams <dan.j.williams@intel.com> wrote:
> 
>> On Tue, Apr 20, 2021 at 5:55 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>>>>
>>>> The dev_dbg() filter language is attractive, it's too bad  
>>>
>>> Not sure what you mean by that. What filter language. Tracepoints do have a
>>> pretty good filtering too.  
>>

> But you can add your own trace point, and even make it generic. That's what
> bpf did for their bpf_trace_printk. You could convert dev_dbg() into a
> tracepoint!
> 
> 
> static __printf(2, 3) int __dev_dbg(const struct device *dev, char *fmt, ...)
> {
> 	static char buf[DEV_DEBUG_PRINTK_SIZE];
> 	unsigned long flags;
> 	va_list ap;
> 	int ret;
> 
> 	raw_spin_lock_irqsave(&dev_dbg_printk_lock, flags);
> 	va_start(ap, fmt);
> 	ret = vsnprintf(buf, sizeof(buf), fmt, ap);
> 	va_end(ap);
> 	/* vsnprintf() will not append null for zero-length strings */
> 	if (ret == 0)
> 		buf[0] = '\0';

Wrong. snprintf(buf, 16, "") will work just fine and cause a '\0' to be
written to buf[0]. As will snprintf(buf, 16, "%s", ""), and any other
case where there ends up being no characters printed.

The only case where snprintf does not guarantee nul-termination is when
the _buffer size_ is 0, in which case vsnprintf obviously cannot and
must not write anything at all (that's used for the "how much do I need
to allocate" situation).

> 
> #define dev_dbg(dev, fmt, ...) 					\
> 	do {							\
> 		if (trace_dev_dbg_printk_enabled())		\
> 			__dev_dbg(dev, fmt, ##__VA_ARGS__);	\
> 	} while (0)
> 
> Note, the "trace_dev_dbg_printk_enabled()" is a static branch, which means
> it is a nop when the dev_dbg_printk tracepoint is not enabled, and is a jmp
> to the __dev_dbg() logic when it is enabled. It's not a conditional branch.

dynamic_debug has been implemented in terms of static_keys for a long
time. And that's a per-dev_dbg invocation static key. IIUC, the above
would cause every single dev_dbg in the kernel to pass through the "grab
a raw spin lock and do the snprintf" thing even when one is just
interested in the dev_dbgs inside a single driver or function.

Rasmus