Messages in this thread | | | Date | Thu, 06 Aug 2009 09:35:39 +0800 | From | Li Zefan <> | Subject | Re: [RFC][PATCH 5/5] tracing/filters: Provide support for char * pointers |
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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.
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