Re: Linking with external lib

[David Smith <dsmith at redhat dot com>]

On 09/07/2016 10:24 AM, William Cohen wrote:
> 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.

I've looked at the kernel's libcfs_nid2str() function a bit. Cloning the
function really isn't going to work. The function uses an internal
lookup table that you won't have access to.

If the nid can't be found in the lookup table, libcfs_nid2str() returns
a default string. Here's a simplified version of libcfs_nid2str() that
shows how it creates the default string:

	__u32 addr = LNET_NIDADDR(nid);
	__u32 net = LNET_NIDNET(nid);
	__u32 nnum = LNET_NETNUM(net);
	__u32 lnd = LNET_NETTYP(net);
	...
	snprintf(buf, buf_size, "%x@<%u:%u>", addr, lnd, nnum);

If that's acceptable, it should be fairly easy to do in systemtap.

-- 
David Smith
dsmith@redhat.com
Red Hat
http://www.redhat.com
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