[Bug libc/13092] New: Profiling on powerpc32 secure-plt shared libs and PIEs segfaults

["amodra at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13092

             Bug #: 13092
           Summary: Profiling on powerpc32 secure-plt shared libs and PIEs
                    segfaults
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: amodra@gmail.com
    Classification: Unclassified


Profiling of shared libraries and PIEs has been broken ever since the
introduction of secure-plt for powerpc32.  The problem is that a ppc32 PIC PLT
call stub uses r30 (GOT pointer) to load the function address from the PLT, r30
being set up in the function prologue, but _mcount is called before the
function prologue.  So chances are good that r30 will be pointing to the
executable GOT when trying to call _mcount in a shared lib function.  A similar
problem can occur in a PIE if a shared lib calls a function in the executable.

Thus it is not possible to call _mcount via a PIC PLT call stub.  Changing the
stub just for _mcount would be possible, but would require followup changes to
exception handling and gdb.  Changing gcc code would either result in bloat to
-pg objects or ABI changes for _mcount, breaking user _mcount implementations.

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