RE: accessing target globals with systemtap

[Jeff Haran <Jeff dot Haran at citrix dot com>]

> -----Original Message-----
> From: Frank Ch. Eigler [mailto:fche@redhat.com]
> Sent: Tuesday, September 11, 2012 5:16 PM
> To: Jeff Haran
> Cc: 'systemtap@sourceware.org'
> Subject: Re: accessing target globals with systemtap
> 
...
> Try instead printing this value from a kernel.function type probe.

Frank,

Thanks a bunch for helping me through this. I did try another kernel.function type probe after reading some related email. I figured arp_init() in net/ipv4/arp.c would have arp_tbl in its scope and it should get called before the functions I am interesting in having the value of arp_tbl in:

171 struct neigh_table arp_tbl = {
...
1235 void __init arp_init(void)
1236 {
1237         neigh_table_init(&arp_tbl);
1238
1239         dev_add_pack(&arp_packet_type);
1240         arp_proc_init();
1241 #ifdef CONFIG_SYSCTL
1242         neigh_sysctl_register(NULL, &arp_tbl.parms, NET_IPV4,
1243                               NET_IPV4_NEIGH, "ipv4", NULL, NULL);
1244 #endif
1245         register_netdevice_notifier(&arp_netdev_notifier);
1246 }

But that yields this:

[root@s01b06 jharan]# cat temp2.stp
global arp_tbl

probe kernel.function("arp_init").call
{
        arp_tbl = &$arp_tbl
}

[root@s01b06 jharan]# stap -v -r 2.6.32-220.el6.x86_64 temp2.stp -m temp2 -p 4
Pass 1: parsed user script and 77 library script(s) using 97284virt/22832res/2912shr kb, in 130usr/10sys/143real ms.
semantic error: no match while resolving probe point kernel.function("arp_init").call
Pass 2: analyzed script: 0 probe(s), 0 function(s), 0 embed(s), 1 global(s) using 249284virt/122356res/79740shr kb, in 410usr/70sys/484real ms.
Pass 2: analysis failed.  Try again with another '--vp 01' option.
[root@s01b06 jharan]#

I also tried this (leaving out  the '&'), just to see if that would change what it was complaining about:

[root@s01b06 jharan]# cat temp2.stp
global arp_tbl

probe kernel.function("arp_init").call
{
        arp_tbl = $arp_tbl
}

[root@s01b06 jharan]# stap -v -r 2.6.32-220.el6.x86_64 temp2.stp -m temp2 -p 4
Pass 1: parsed user script and 77 library script(s) using 97284virt/22836res/2912shr kb, in 120usr/10sys/139real ms.
semantic error: no match while resolving probe point kernel.function("arp_init").call
Pass 2: analyzed script: 0 probe(s), 0 function(s), 0 embed(s), 1 global(s) using 249288virt/122360res/79740shr kb, in 400usr/70sys/463real ms.
Pass 2: analysis failed.  Try again with another '--vp 01' option.
[root@s01b06 jharan]#

>From the error messages, it seems like it's having problems with finding arp_init() rather than any particular problem with the probe body.

Is this because arp_init is declared with __init?

I also tried kernel.statement instead:

[root@s01b06 jharan]# cat temp3.stp
global arp_tbl

probe kernel.statement("*@net/ipv4/arp.c:1238")
{
        arp_tbl = $arp_tbl
}
[root@s01b06 jharan]# stap -v -r 2.6.32-220.el6.x86_64 temp3.stp -m temp3 -p 4
Pass 1: parsed user script and 77 library script(s) using 97284virt/22832res/2912shr kb, in 130usr/0sys/139real ms.
semantic error: no match while resolving probe point kernel.statement("*@net/ipv4/arp.c:1238")
Pass 2: analyzed script: 0 probe(s), 0 function(s), 0 embed(s), 1 global(s) using 267124virt/76472res/17420shr kb, in 170usr/80sys/238real ms.
Pass 2: analysis failed.  Try again with another '--vp 01' option.
[root@s01b06 jharan]#

I think I have that kernel.statement syntax right. This one did work as expected and it looks pretty much the same:

probe kernel.statement("*@net/core/neighbour.c:934")
...

It seems like it just can't find that arp_init code, regardless of the probe body.

Is there something I need to do in order to roll kernel.function or kernel.statement in order to probe arp_init()?

Also, what is the recommended way of getting the address of an in-scope structure instance?

As you've pointed out this is wrong:

        arp_tbl = $arp_tbl

Is this the way to do it?

        arp_tbl = &$arp_tbl

> > Is there a way for me to get access to a kernel global in my version of stap?
> 
> Another way would be to use embedded-C:
> 
> %{
> #include <net/arp.h>
> %}
> function the_table_address:long () %{ /* unmangled */
>   /* rely on EXPORT_SYMBOL ... to let this resolve */
>   THIS->__retvalue = & arp_tbl;
> %}
> probe begin {
>   // to find the base address
>   printf("%p\n", the_table_address())
>   // to fetch fields:
>   printf("%d\n", @cast(the_table_address(),"neigh_table","kernel")-
> >family);
> }

I'll give this embedded C a try.

Thanks again,

Jeff Haran