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

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Date:	Sun, 31 May 2009 16:27:42 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Tom Zanussi <tzanussi@...il.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tracing/filters: use strcmp() instead of strncmp()

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@...fujitsu.com>:
>>>> 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.

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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