Re: [RFC][PATCH 5/5] tracing/filters: Provide support for char * pointers

[Li Zefan <lizf@cn.fujitsu.com>]

Frederic Weisbecker wrote:
> On Mon, Aug 03, 2009 at 02:58:15PM +0800, Li Zefan wrote:
>> Frederic Weisbecker wrote:
>>> Provide support for char * pointers in the filtering framework.
>>> Usually, char * entries are dangerous in traces because the string
>>> can be released whereas a pointer to it can still wait to be read from
>>> the ring buffer. But sometimes we can assume it's safe, like in case
>>> of RO data (eg: __file__ or __line__, used in bkl trace event). If
>>> these RO data are in a module and so is the call to the trace event,
>>> then it's safe, because the ring buffer will be flushed once this
>>> module get unloaded.
>>>
>> The problem is we don't distinguish dangerous char * from
>> safe char *... They are both defined as:
>> 	__field(char *, str)
>>
>> So for those dangerous ones, a string filter still can be applied,
>> which will dereference those pointers.
> 
> Yeah, but only reviewing can distinguish them. It depends on the
> context.
> IMO, a __builtin_constant check would be wrong. I don't remember who
> posted recently tracepoints with char * types that were safe although he
> didn't use string constants.
> 

IMO it's really bad to rely on review to prevent wrong use of
an API..

Other developers won't know this restriction, and not all tracepoint
patches go through -tip tree, and not all trace_event source files
are in include/trace/events/.

How about add __field_type()? So we can define:

	__field_type(char *, str, FILTER_PTR_STR)

the advantage is he who wrote the code really knows this field is safe
to be used in filtering as a string.

I had some patches that does similar job. I can rewrite and post them.