Re: Linking with external lib

[William Cohen <wcohen at redhat dot com>]

On 09/06/2016 03:39 PM, Mahmoud Hanafi wrote:
> 
> 
> On 09/06/2016 12:34 PM, David Smith wrote:
>> On 09/06/2016 02:08 PM, Mahmoud Hanafi wrote:
>>>
>>> On 09/06/2016 11:55 AM, David Smith wrote:
>>>> On 09/06/2016 01:37 PM, Mahmoud Hanafi wrote:
>>>>> In my stap script I have some embedded C function that needs to make
>>>>> some calls to external libraries. Is there I can link my script with the
>>>>> external libs? Or do i need to add all the code I need into my script.
>>>> Basically you can't. A systemtap script gets compiled into a kernel
>>>> module (assuming you are using the default 'linux' runtime). A kernel
>>>> module can't access *any* user library.
>>>>
>>>> Perhaps you should back up a bit and let us know what your external
>>>> library is trying to do.
>>>>
>>> The library is part of lustre. I need the functions that does address
>>> translation to string values.
>> Can you point me to the library/function on lustre.org that you are
>> interested in?
>>
>> You can't really copy userspace C code into systemtap. Systemtap tries
>> very hard to handle memory accesses safely (since otherwise you can
>> crash the kernel). We might be able to "translate" the userspace C code
>> into embedded C.
> the function is defined here
> http://osxr.org:8080/linux/source/drivers/staging/lustre/lustre/libcfs/nidstrings.c#0382
> 
> "libcfs_nid2str"
> 
> It would be nice to compile in something like this, because if there are changes later in the function it wouldn't require stap script changes.
> 
> 

Hi,

Isn't libcfs_nid2str a kernel function?  For some kernel functions one could use systemtap guru mode to directly call c functions in the kernel.  However, it doesn't look like this particular function is visible to code outside the lustre module.

You might be able to include <linux/libcfs/libcfs.h> and clone the libcfs_nid2str function using guru mode (-g) as a work around. At least that would allow access to the various defines and macros.  Use the systemtap/tapset/linux/tcp.stp as an example of how to do the include and systemtap functions (https://www.sourceware.org/git/?p=systemtap.git;a=blob;f=tapset/linux/tcp.stp;h=22fe2f08fda55c989d33cdcd776fa00736f4b7dd;hb=HEAD).  Not an ideal solution, but it might get you closer to what you want.

-Will