JDK-8019929 : PPC64 (part 107): Extend ELF-decoder to support PPC64 function descriptor tables
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: port-stage-ppc-aix
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • OS: linux
  • CPU: ppc
  • Submitted: 2013-07-04
  • Updated: 2016-10-24
  • Resolved: 2013-12-06
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JDK 8 JDK 9 Other
8u20Fixed 9Fixed openjdk7u,port-stage-ppc-aixFixed
This is preparation for PPC64 integration: http://openjdk.java.net/jeps/175 
This and following ppc64 changes will go into staging repository first and tested there: http://hg.openjdk.java.net/ppc-aix-port/stage/ 


On PowerPC-64 (and other architectures like for example IA64) a pointer to
a function is not just a plain code address, but instead a pointer to a so
called function descriptor (which is simply a structure containing 3
pointers). This fact is also reflected in the ELF ABI for PowerPC-64.

On architectures like x86 or SPARC, the ELF symbol table contains the start
address and size of an object. So for example for a function object (i.e.
type 'STT_FUNC') the symbol table's 'st_value' and 'st_size' fields
directly represent the starting address and size of that function. On PPC64
however, the symbol table's 'st_value' field only contains an index into
another, PPC64 specific '.opd' (official procedure descriptors) section,
while the 'st_size' field still holds the size of the corresponding
function. In order to get the actual start address of a function, it is
necessary to read the corresponding function descriptor entry in the '.opd'
section at the corresponding index and extract the start address from

That's exactly what this 'ElfFuncDescTable' class is used for. If the
HotSpot runs on a PPC64 machine, and the corresponding ELF files contains
an '.opd' section (which is actually mandatory on PPC64) it will be read
into an object of type 'ElfFuncDescTable' just like the string and symbol
table sections. Later on, during symbol lookup in
'ElfSymbolTable::lookup()' this function descriptor table will be used if
available to find the real function address.

All this is how things work today (2013) on contemporary Linux
distributions (i.e. SLES 10) and new version of GCC (i.e. > 4.0). However
there is a history, and it goes like this:

In SLES 9 times (sometimes before GCC 3.4) gcc/ld on PPC64 generated two
entries in the symbol table for every function. The value of the symbol
with the name of the function was the address of the function descriptor
while the dot '.' prefixed name was reserved to hold the actual address of
that function (

For a C-function 'foo' this resulted in two symbol table entries like this
(extracted from the output of 'readelf -a '):

Section Headers:
  [ 9] .text             PROGBITS         0000000000000a20  00000a20
       00000000000005a0  0000000000000000  AX       0     0     16
  [21] .opd              PROGBITS         00000000000113b8  000013b8
       0000000000000138  0000000000000000  WA       0     0     8

Symbol table '.symtab' contains 86 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
    76: 00000000000114c0    24 FUNC    GLOBAL DEFAULT   21 foo
    78: 0000000000000bb0    76 FUNC    GLOBAL DEFAULT    9 .foo

 You can see now that the '.foo' entry actually points into the '.text'
segment ('Ndx'=9) and its value and size fields represent the functions
actual address and size. On the other hand, the entry for plain 'foo'
points into the '.opd' section ('Ndx'=21) and its value and size fields are
the index into the '.opd' section and the size of the corresponding '.opd'
section entry (3 pointers on PPC64).

These so called 'dot symbols' were dropped around gcc 3.4 from GCC and
BINUTILS, see http://gcc.gnu.org/ml/gcc-patches/2004-08/msg00557.html. But
nevertheless it may still be necessary to support both formats because we
either run on an old system or because it is possible at any time that
functions appear in the stack trace which come from old-style libraries.

Therefore we not only have to check for the presence of the function
descriptor table during symbol lookup in 'ElfSymbolTable::lookup()'. We
additionally have to check that the symbol table entry references the
'.opd' section. Only in that case we can resolve the actual function
address from there. Otherwise we use the plain 'st_value' field from the
symbol table as function address. This way we can also lookup the symbols
in old-style ELF libraries (although we get the 'dotted' versions in that
case). However, if present, the 'dot' will be conditionally removed on
PPC64 from the symbol in 'ElfDecoder::demangle()' in decoder_linux.cpp.

Notice that we can not reliably get the function address from old-style
libraries because the 'st_value' field of the symbol table entries which
point into the '.opd' section denote the size of the corresponding '.opd'
entry and not that of the corresponding function. This has changed for the
symbol table entries in new-style libraries as described at the beginning
of this documentation.

This change also slightly improves the implementation of
ElfSymbolTable::lookup(). Before, the method always iterated over all
symbols in the symbol table and returned the one with the highest address
below the requested addr argument. This not only could take a significant
amount of time for big libraries, it could also return bogus symbols for
addresses which were not really covered by that symbol table at all. The
new versions additionally uses the symbol table's st_size field to verify
that the requested addr argument is indeed within the range covered by the
corresponding symbol table entry. If so, the search is stopped and the
symbol is returned immediately.