PSARC/2008/179 cross link-editor

[Ali Bahrami]

I am sponsoring the following self-reviewed case for myself.

I believe it qualifies for self review: The changes have limited
scope, are similar to existing features, and preserve full backward
compatibility.

Release Binding:			Patch/Micro
'-z target=' ld option:			Committed
DT_SUNW_LDMACH dynamic tag		Committed

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Overview
--------

The integration of

	6671255 link-editor should support cross linking

will extend the Solaris link-editor from being a purely native
linker, to a cross linker. This means that a linker running
on any Solaris host will be able to link objects for any other
supported target system. We see two benefits:

	- Simplify porting of OpenSolaris to new architectures.

	  The Solaris code makes heavy use of the unique features
	  supported by Solaris link-editor, many of which were
	  developed for just that purpose. Ports of OpenSolaris
	  usually attempt to bootstrap using the GNU ld, which
	  already supports cross linking, but which does not support
	  all the Solaris features we use. The ability to add a new
	  target to the Solaris linker using a non-native host, and then
	  to use that linker to bootstrap the rest of the system is
	  a significant shortcut.

	- Allows the use of fast/cheap commodity desktop systems to
	  build objects for other systems, be they large expensive
	  systems, or small embedded devices.



New ld command option: -z target=XXX
------------------------------------

Traditionally, ld establishes the class (whether the object is 32
or 64-bit) of the object being generated by examining the first ELF
object processed from the command line. In the case where an object
is being built solely from an archive library or a mapfile, the -64
command line option is available to explicitly override this default.
We extend this mechanism to also determine the machine type of the
object to produce:

	- The class and machine type of the resulting object
	  default to those of the first ELF object processed
	  from the command line.

	- The new '-z target=' command line option can be
	  used to explicitly specify the machine type.
	  Supported target types will be 'sparc', and 'x86'.



New ELF Dynamic Section tag: DT_SUNW_LDMACH
-------------------------------------------

A cross link-editor has to contend with byte order and alignment
issues not encountered by a native link-editor. Errors in the code
that handles this can lie latent in a native linker and only cause
problems when the linker host and target have different requirements.
When debugging linker issues, it will be important to be able to
reliably determine the  machine type of the linker that produced
the object.

The link-editor will tag every dynamic object it produces with a
DT_SUNW_LDMACH dynamic section entry. DT_SUNW_LDMACH is an integer
containing the EM_* ELF machine type code for the linker that produced
the object. This information is not used by the runtime linker, and
exists purely as documentation. It is easily displayed via dump,
elfdump, or elfedit. For example, given a sparc a.out binary
built using the Solaris X86 cross linker:

	% elfdump a.out | grep EM_
	  e_machine:  EM_SPARC32PLUS      e_version:    EV_CURRENT
	      [27]  SUNW_LDMACH       0x3                 EM_386

Even in a native linking situation, the presence of SUNW_LDMACH
can be informative, as it allows us to determine whether the 32-bit
or 64-bit version of the link-editor was employed.