[Bug translator/11097] New: debug memory tracker shows memory overwrite in MAXNESTING

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

After getting the debug memory tracker (activated with -DDEBUG_MEM) going again,
it found a problem with the following testcase (part of
src/testsuite/systemtap.base/control_limits.exp):

====
# stap -u -DMAXNESTING=3 ../src/testsuite/systemtap.base/control_limits.stp
ERROR: MAXNESTING exceeded near identifier 'recurse' at
../src/testsuite/systemtap.base/control_limits.stp:3:10
WARNING: Number of errors: 1, skipped probes: 0
====

Note that the above testcase is designed to fail, so the error printed there is OK.

When run, the debug memory tracker uses printk to print the following on the
console:

====
Dec 15 17:02:02 kvm-rawhide-64-1 kernel: SYSTEMTAP ERROR: Memory fence corrupted
after allocated memory
Dec 15 17:02:02 kvm-rawhide-64-1 kernel: at addr ffff88003e476698. (Allocation
ends at ffff88003e476697)
====

Since the memory fence (a small bit of memory placed after every allocation) has
been overwritten, this means systemtap is writing past the end of the allocated
memory.

After a bit of debugging, this appears to be happening to the allocated context
structure.

-- 
           Summary: debug memory tracker shows memory overwrite in
                    MAXNESTING
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: translator
        AssignedTo: systemtap at sources dot redhat dot com
        ReportedBy: dsmith at redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=11097

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