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

From: Li Zefan
Date: Sat May 30 2009 - 05:05:34 EST

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?

> Thanks,
> Frederic.
>> Since __string() is dynamic size, we are not able to set field->size to
>> string length. Thus this patch uses strcmp() instead of strncmp().
>> [ Impact: make filter facility working normally for __string() field ]
>> Signed-off-by: Li Zefan <lizf@xxxxxxxxxxxxxx>
>> ---
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