Messages in this thread | | | Date | Thu, 6 Aug 2009 03:59:50 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFC][PATCH 5/5] tracing/filters: Provide support for char * pointers |
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On Thu, Aug 06, 2009 at 09:35:39AM +0800, Li Zefan wrote: > 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.
Ah good idea. That may even be useful for further typedef'ed types which filter process match existing supported types.
Just one neat however: __field_type looks too much ambiguous. __field() is already here to define a typed field. This seems confusing.
Why not __field_ext() for "extended"? We may probably add more flags than FILTER_PTR_STR in the future to define options for filtering or even for larger scope.
I then wait for your patches to be posted and I'll integrate them in the current queue.
Thanks a lot!
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