Re: [PATCH 2/2] tracing/filters: use strcmp() instead of strncmp()

[Frederic Weisbecker]

On Sun, May 31, 2009 at 04:27:42PM +0800, Li Zefan wrote:
> Frederic Weisbecker wrote:
> > On Sat, May 30, 2009 at 05:06:39PM +0800, Li Zefan wrote:
> >> Frédéric Weisbecker wrote:
> >>> 2009/5/29 Li Zefan <lizf@xxxxxxxxxxxxxx>:
> >>>> Trace filter is not working normally:
> >>>>
> >>>>  # echo 'name == et' > tracing/events/irq/irq_handler_entry/filter
> >>>>  # echo 1 > tracing/events/irq/irq_handler_entry/enable
> >>>>  # cat trace_pipe
> >>>>      <idle>-0     [001]  1363.423175: irq_handler_entry: irq=18 handler=eth0
> >>>>      <idle>-0     [001]  1363.934528: irq_handler_entry: irq=18 handler=eth0
> >>>>      ...
> >>>>
> >>>> It's because we pass to trace_define_field() the information of
> >>>> __str_loc_##item, but not the actual string, so pred->str_len == field->size
> >>>> == sizeof(unsigned short), thus it always compare at most 2 bytes when
> >>>> filtering on __string() field.
> >>>
> >>> Weird, I was about sure I set the size of each string() to FILTER_MAX_STRING (or
> >>> something like that).
> >>>
> >>> Anyway this patch looks good but it does more than just fixing the
> >>> issue, it removes
> >>> the string len boundary security we had with strncmp() for every
> >>> string (static and
> >>> dynamic size).
> >>>
> >>> The potential side effect that comes along this patch would disappear if
> >>> you just turn strncmp into strcmp only in filter_pred_strloc().
> >>>
> >>> If you do that also for fixed size strings, then it should be done in
> >>> a second patch,
> >>> although I guess turning anything here into strcmp is fine because the
> >>> strings given
> >>> by the user are always limited in their size. But we never know...
> >>>
> >> I don't think there's any security issue. It's irrelevant how big the user-input
> >> strings are. The point is those strings are guaranteed to be NULL-terminated.
> >> Am I missing something?
> >>
> >> And I don't think it's necessary to make 2 patches that each patch converts
> >> one strncmp to strcmp. But maybe it's better to improve this changelog?
> > 
> > Hmm, you must be right, indeed they seem to be guaranted beeing NULL-terminated
> > strings.
> > 
> 
> Sorry, I was wrong. :(
> 
> Though the user-input strings are guaranted to be NULL-terminated, strings
> generated by TRACE_EVENT might not.
> 
> We define static strings this way:
> 	TP_struct(
> 		__array(char, foo, LEN)
> 	)
> But foo is not necessarily a string, though I doubt someone will use it
> as non-string char array.


Yeah, but the user defined comparison operand is NULL terminated.
So the strcmp will stop at this boundary.


 
> Dynamic string is fine, because assign_str() makes it NULL-terminated.
> 
> So we can use strcmp() for dynamic strings, but we'd better use strncmp() for
> static string.
> 
> 

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