[Bug tapsets/14296] New: The blk.stp example fails occasionally

["dsmith at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14296

             Bug #: 14296
           Summary: The blk.stp example fails occasionally
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tapsets
        AssignedTo: systemtap@sourceware.org
        ReportedBy: dsmith@redhat.com
    Classification: Unclassified


Occasionally the blk.stp example gets an error.  Here the output found in the
logfile:

====
 attempting command stap bkl.stp -c "sleep 0.2" 1
OUT ERROR: kernel string copy fault at 0x0000000000000318 near identifier
'kernel_string' at /usr/local/share/systemtap/tapset/conversions.stp:18:10
stap ready
====

That error comes from tapset task_execname() function, which looks like this:

function task_execname:string (task:long)                                       
{                                                                               
    return kernel_string(@cast(task, "task_struct",
"kernel<linux/sched.h>")->c\
omm)                                                                            
}                                                                               

What happens here is that if 'task' is NULL, we try to read the comm string
from address 0x318, which is the offset of the comm field in the task_struct.

Either blk.stp needs to quit passing NULL to task_execname() or task_execname()
needs to handle a passed in NULL.

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