[lttng-dev] How to ensure null char to string field?

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Mon May 28 11:27:31 EDT 2012

* Francis Giraldeau (francis.giraldeau at gmail.com) wrote:
> Hi, 
> I want to save a string from user space through a proc file. It seems
> trivial, but I have a problem with null termination. The user space
> string is _not_ null terminated. Here is an hexdump of the string bytes
> for "bidon" after performing copy_from_user on the kernel stack:
> [ 3163.957414] 0x62 0x69 0x64 0x6F 0x6E 0xFFFFFFDF ...
> The tracepoint I use is:
> TRACE_EVENT(lttng_uevent_cfu,
> 	TP_PROTO(const char * str),
> 	TP_ARGS(str),
> 	TP_STRUCT__entry(__string_from_user(str, str)),
> 	TP_fast_assign(tp_copy_string_from_user(str, str)),
> 	TP_printk()
> )
> The user_strlen continues beyond the real string length because it's not
> null terminated. The string length is already known (given as a
> parameter of the write callback), such that I would like to copy only
> this number of bytes and then set last byte to '\0'. Also, this would be
> required to limit the string size.
> I have not found a way to do it with current macros. Shall a new macro
> to handle this use case be added?

I think we would need to add:

for TP_STRUCT__entry:


and implement, for TP_fast_assign:


which will write the string length, followed by an array of bytes,
copied from userspace.

Look at all the "__dynamic_array" and "tp_memcpy_dyn" match in
lttng-events.h as a starting point.



> Cheers,
> Francis Giraldeau
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> lttng-dev at lists.lttng.org
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Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.

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