Messages in this thread | | | Date | Fri, 07 Aug 2009 11:12:53 +0800 | From | Li Zefan <> | Subject | Re: [PATCH 3/3] tracing/filters: Support filtering for char * strings |
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Steven Rostedt wrote: > On Fri, 7 Aug 2009, Li Zefan wrote: > >>>> if (is_string_field(field)) { >>>> + pred->str_len = field->size; >>>> + >>>> if (field->filter_type == FILTER_STATIC_STRING) >>>> fn = filter_pred_string; >>>> - else >>>> + else if (field->filter_type == FILTER_DYN_STRING) >>>> fn = filter_pred_strloc; >>>> - pred->str_len = field->size; >>>> + else { >>>> + fn = filter_pred_pchar; >>>> + pred->str_len = strlen(pred->str_val); >>>> + } >>> I'm a little dense here, where do we protect against someone making a >>> tracepoint that points to unsafe data? >>> >> We can't prevent anyone from doing insane things deliberately, but >> we prevent from doing wrong things unconsciously. >> >> Only if a TRACE_EVENT has a field defined as: >> >> __field_ext(char *, name, FILTER_PTR_STR) >> >> Here using FILTER_PTR_STR explicitly, he should know what he's doing. >> >> Anyway, he can make a ptr pointing to unsafe data this way: >> >> TP_STRUCT__entry( >> __field(char *, name) >> ) >> TP_printk("%s", name) > > I guess the thing I'm missing is what's the difference of the two? Why > would a developer use __field_ext instead of doing it the unsafe way of > just __field? > > I guess I don't see the developer doing something wrong unconsciously. > Well maybe I don't see this making the developer do it right > unconsciously. > > What protection is this giving us? >
__field(char *) suggests it should be treated as plain pointer, while __field_ext(char *, FILTER_PTR_STR) suggests he's aware it's safe to dereference the pointer, for example the case in Frederic's blk events.
In Frederic's initial version, "char *" field will always be attached to ptr_str filter function. This is unsafe, because for other fields defined as "char *" but not safe to dereference, a user still can do this:
# echo 'name == abc' > filter
Then we'll deref a pointer that can point to unsafe data.
In this patch, this won't happen, as long as the developer is aware that his use of __field_ext(char *) is right.
Otherwise, he will just use normal __field(char *) and print the pointer itself in TP_printk().
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