linux-kernel - Re: [PATCH] tracing/uprobes: Fix output for multiple string arguments

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Date:   Wed, 16 Jan 2019 15:13:37 +0100
From:   Andreas Ziegler <andreas.ziegler@....de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing/uprobes: Fix output for multiple string arguments

On 1/16/19 2:40 PM, Steven Rostedt wrote:
> On Wed, 16 Jan 2019 10:41:12 +0100
> Andreas Ziegler <andreas.ziegler@....de> wrote:
> 
>> When printing multiple uprobe arguments as strings the output for the
>> earlier arguments would also include all later string arguments.
>>
>> This is best explained in an example:
>>
>> Consider adding a uprobe to a function receiving two strings as
>> parameters which is at offset 0xa0 in strlib.so and we want to print
>> both parameters when the uprobe is hit (on x86_64):
>>
>> $ echo 'p:func /lib/strlib.so:0xa0 +0(%di):string +0(%si):string' > \
>>      /sys/kernel/debug/tracing/uprobe_events
>>
>> When the function is called as func("foo", "bar") and we hit the probe,
>> the trace file shows a line like the following:
>>
>>    [...] func: (0x7f7e683706a0) arg1="foobar" arg2="bar"
>>
>> Note the extra "bar" printed as part of arg1. This behaviour stacks up
>> for additional string arguments.
>>
>> The strings are stored in a dynamically growing part of the uprobe
>> buffer by fetch_store_string() after copying them from userspace via
>> strncpy_from_user(). The return value of strncpy_from_user() is then
>> directly used as the required size for the string. However, this does
>> not take the terminating null byte into account as the documentation
>> for strncpy_from_user() cleary states that it "[...] returns the
>> length of the string (not including the trailing NUL)" even though the
>> null byte will be copied to the destination.
>>
>> Therefore, subsequent calls to fetch_store_string() will overwrite
>> the terminating null byte of the most recently fetched string with
>> the first character of the current string, leading to the
>> "accumulation" of strings in earlier arguments in the output.
>>
>> Fix this by incrementing the return value of strncpy_from_user() by
>> one if we did not hit the maximum buffer size.
>>
>> Signed-off-by: Andreas Ziegler <andreas.ziegler@....de>
>> ---
>>   kernel/trace/trace_uprobe.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
>> index e335576b9411..dfb9bbc7fd82 100644
>> --- a/kernel/trace/trace_uprobe.c
>> +++ b/kernel/trace/trace_uprobe.c
>> @@ -160,6 +160,13 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
>>   	if (ret >= 0) {
>>   		if (ret == maxlen)
>>   			dst[ret - 1] = '\0';
>> +		else if (ret > 0)
> 
> Do we need the ret > 0 check? What if the value is ""?
> 
> Doesn't that cause the same issue?
> 
> -- Steve
> 

yes, it does. With this patch an empty string will also print "(fault)", 
I missed that, sorry. I'll send a v2.

Thanks,

Andreas

>> +			/*
>> +			 * Include the terminating null byte. In this case it
>> +			 * was copied by strncpy_from_user but not accounted
>> +			 * for in ret.
>> +			 */
>> +			ret++;
>>   		*(u32 *)dest = make_data_loc(ret, (void *)dst - base);
>>   	}
>>   
> 


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