xref: /illumos-gate/usr/src/cmd/sgs/libld/common/README.XLINK (revision 7e16fca0)

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Notes On Cross Link-Editor Support in libld.so
-----------------------------------------

The Solaris link-editor is used in two contexts:

	1) The standard ld command
	2) Via the runtime linker (ld.so.1), when a program
	   calls dlopen() on a relocatable object (ET_REL).

To support both uses, it is packaged as a sharable library (libld.so).
The ld command is therefore a simple wrapper that uses libld.

libld.so is a cross linker. This means that it can link objects for
a system other than the system running the link-editor (e.g. A link-editor
running on an amd64 system processing sparc objects). This means that every
instance of libld.so contains code for building objects for every supported
target. It is not necessary to build libld specifically for the
platform you're targeting. This is possible because we only support
Solaris/ELF, with a small number of platforms, and the additional code
required per target is small.

At initialization, the caller of libld.so specifies the type of objects
being linked. By default, the ld command determines the machine type and
class of the object being generated from the first ELF object processed
from the command line. The -64 and -ztarget options exists to change this
default, which is useful when creating an object entirely from an archive
library or a mapfile. During initialization, the link-editor configures
itself to build an output object of the specified type. This is done via
indirection, using the global ld_targ structure to access code, data, and
constants for the specified target.

There are two types of source files used to build libld.so:

	1) Common code used for all targets
	2) Target specific code used only when linking for
	  a given target.

All of these files reside in usr/src/cmd/sgs/libld/common. However,
it is easy to see which files belong in each category by examining
the object lists maintained in usr/src/cmd/sgs/libld/Makefile.com.
In addition, the target-specific files usually include the target
in their name (i.e. machrel.sparc.c).

Although the target dependent and independent (common) code is well separated,
they are interdependent. The common code is explicitly aware of
target-specific section types that can occur only for some targets
(i.e. SHT_AMD64_UNWIND). This is not an architecture that allows
for arbitrary target support to be dynamically plugged into an unchanged
platform independent core. Rather, it is an organization that allows
a common core to support all the targets it knows about in a way that
is understandable and maintainable. A truly pluggable architecture
would be considerably more opaque and complex, and is neither necessary,
nor desirable, given the wide commonality between modern computer
architectures.

It is possible to add support for new targets to libld.so. The process
of doing so is largely a matter of examining the files for existing
platforms, studying the ABI for the new target platform, and then
filling in the missing pieces for the new target. The remainder of this
file consists of sections that describe some of the issues and steps
that you will encounter in adding a new target.

-----------------------------------------------------------------------------
The relocation code used by ld is shared by the runtime linker (ld.so.1)
and by the kernel module loader (ktrld), and is therefore found under
usr/src/uts. You must add code for a relocation engine to support the
new target. To do so, examine the common header files:

	usr/src/uts/common/krtld/reloc.h
	usr/src/uts/common/krtld/reloc_defs.h

   and the existing relocation engines:

	usr/src/uts/intel/amd64/krtld/doreloc.c
	usr/src/uts/intel/ia32/krtld/doreloc.c
	usr/src/uts/sparc/krtld/doreloc.c

The ABI for the target platform will be the primary information
you require. If your new system has attributes not found in an existing
target, you may have to add/modify fields in the Rel_entry struct typedef
(reloc_defs.h), or you may have to add new flags. Either sort of change
may require you to also modify the existing relocation engines, and
undoubtedly the common code in libld.so as well.

When compiled for use by libld, the relocation engine requires an
argument named "bswap". Each relocation engine must be prepared to
swap the data bytes it is operating on. This support allows a link-editor
running on a platform with a different byte order than the target to
build objects for that target. To see how this is implemented, and how
to ifdef that support so it only exists in the libld version of
the engine, examine the code for the existing engines.

-----------------------------------------------------------------------------
You must create a target subdirectory in usr/src/cmd/sgs/include,
and construct a machdep_XXX.h file (where XXX is the name of the
target). The machdep files for the current platforms can be helpful:

	usr/src/cmd/sgs/include/sparc/machdep_sparc.h
	usr/src/cmd/sgs/include/i386/machdep_x86.h

Note that these files support both the 32 and 64-bit versions of
a given platform, so there is only one subdirectory and machdep
file for each platform (i.e. "sparc", instead of "sparc" and "sparcv9").

Once you have created the target machdep_XXX.h file, you must edit:

	usr/src/cmd/sgs/include/machdep.h

and add a #include for your new file to it, surrounded by the
appropriate #ifdef for the target platform.

This two level structure allows us to #include machdep information
in two different ways:

	1) Code that wants support for the current platform,
	   regardless of which platform that is, can include
	   usr/src/cmd/sgs/include/machdep.h. The runtime linker
	   (ld.so.1) is the primary consumer of this form.

	2) Code that needs to support multiple targets must never
	   include the generic machdep.h from (1) above. Instead,
	   such code explicitly includes the machdep file for the target
	   it is interested in. For example:

		#include <sparc/machdep_sparc.h>

	   libld.so uses this form to build non-native target
	   code.

You will find that most of the constants defined in the target
machdep file end up as initialization for the information that
libld.so accesses via the ld_targ global variable.

-----------------------------------------------------------------------------
Study the definition of the Target typedef in

	usr/src/cmd/sgs/libld/common/_libld.h

This is the type of the ld_targ global variable. Filling in a complete
copy of this definition is the primary task involved in adding support
for a new target to libld.so, so it will be helpful to be familiar with
it before you dive deeper. libld follows two simple rules with regards
to ld_targ, and the Target type:

	1) The target-independent common code can only access
	   target-dependent code or data via the ld_targ global
	   variable.

	2) The target-dependent code can access the common
	   code or data freely.

A given link-editor invocation is always for a single target. The choice
of target is made at initialization, and does not change within a
single link. Code for the other targets is present, but is not
accessed.

-----------------------------------------------------------------------------
Files containing the target-specific code to support the new
platform must be added to libld.so. Examine the object lists
in usr/src/cmd/sgs/libld/Makefile.com to see the files for existing
platforms, and read those files to get a sense of what is required.

Among the other files, every platform will have a file named
machrel.XXX.c. This file contains the relocation-related functions,
and it also contains an init function for the target. This init function
is responsible for initializing the ld_targ global variable so that
the common code will use the code and definitions for your
target.

You should start by creating a machrel.XXX.c file for your new
target. Add other files as needed. Be aware that any functions or
variables you place in these target-dependent files must either
be static, or must have names that will not collide with the names
used by the rest of libld.so. The easiest way to do this is to
add a target suffix to the end of all such non-local names
(i.e. foo_sparc() instead of foo()).

The existing code is the documentation for this phase of things: The
best way to determine what a given function should do is to read the
code for other platforms, taking into account the similarities and
differences in the ABI for your new target and those existing ones.

-----------------------------------------------------------------------------
You may find that your new target requires support for new concepts
not found in other targets. A common example of this might be
a new target specific ELF section type (i.e. SHT_AMD64_UNWIND). Another
might be details involving PIC code and PLT generation (as found for
sparc). It may be necessary to add new fields to the ld_targ global
variable, and to modify the libld.so common code to use these new
fields.

It is a standard convention that NULL function pointers are used to
represent functionality not required by a given target. Although the
common code can consult ld_targ.t_m.m_mach to determine the target it
is linking for, and although there is some code that does this, it
is generally undesirable and unnecessary. Instead, the common code
should test for such pointers, as with this sparc-specific example
from update.c:

	/*
	 * Assign a got offset if necessary.
	 */
	if ((ld_targ.t_mr.mr_assign_got != NULL) &&
	    (*ld_targ.t_mr.mr_assign_got)(ofl, sdp) == S_ERROR)
		return ((Addr)S_ERROR);

It may be tempting to include information in the comment that explains
the target specific nature of this, and that may even be appropriate.
Consider however, that a new target may come along with the same feature
later, and if that occurs, your comments will instantly be dated. In general,
the use of ld_targ is a strong hint to the reader that they should go read
the target-specific code referenced to understand what is going on. It is
best to supply comments at the call site that describe the operation
in generic terms (i.e. "assign a got if necessary") instead of in
explicit target terms (i.e. "Assign a sparc got if necessary"). Of
course, some features are extremely target-specific (like amd64 unwind
sections), and it doesn't really help to be obscure in such cases.
This is a judgement call.

If you do add a new field to ld_targ that uses NULL to represent
an option feature *YOU MUST DOCUMENT IT AS SUCH*. You will find
comments in _libld.h for existing optional fields. It suffices to
add a comment for your new field. In the absence of such a comment,
the common code assumes that all function pointers are safe to call
through (dereference) without first testing them.

-----------------------------------------------------------------------------
Byte swapping is a big issue in cross linking, as the system running
the link-editor may have the opposite byte order from the target. It is
important to know when, and when not, to swap bytes.

If the build system and target have different byte orders, the
FLG_OF1_ENCDIFF bit of the ofl_flags field of the output file
descriptor will be set. If this bit is not set, the target and
system byte orders are the same, and no byte swapping
is required.

libld uses libelf to read and write objects. libelf automatically
swaps bytes for the sections it knows about, such as symbol tables,
relocation records, and the usual ELF plumbing. It is therefore never
necessary for your code to swap the bytes in this data. If you find that
this is not the case, you have probably uncovered a bug in libelf that
you should look into.

The big exception to libelf transparently handling byte swapping is
progbits sections (SHT_PROGBITS). libelf does not understand program
code or data as anything other than a series of byte values, and as such,
cannot do byte swapping for them. If your code examines or modifies
such data, you are responsible for handling the byte swapping required.

The OFL_SWAP_RELOC macros found in _libld.h can be helpful in making such
determinations. You should use these macros instead of writing your own
tests for this, as they have high documentation value. If you find they
don't handle your target, add a new one that does.

GOT and PLT sections are SHT_PROGBITS. You will probably find
that the vast majority of byte swapping you have to handle
concern the handling of these items.

libld contains generic functions for byte swapping:

	ld_bswap_Word();
	ld_bswap_Xword();

These functions are built on top of the of the BSWAP_ macros found
in usr/src/cmd/sgs/include/_machelf.h:

	BSWAP_HALF
	BSWAP_WORD
	BSWAP_XWORD

When copying data from one address to another in a cross link environment,
the source and/or destination addresses may not have adequate alignment for
the data type being copied. For example, a sparc platform cannot access
8-byte data types on 4-byte boundaries, but it might need to do so when
linking X86 objects where the alignment of such data can be 4. The
UL_ASSIGN macros can be used to copy potentially unaligned data:

	UL_ASSIGN_HALF
	UL_ASSIGN_WORD
	UL_ASSIGN_XWORD

The UL_ASSIGN_BSWAP macros do unaligned copies, and also perform
byte swapping when the linker host and target byte orders are
different:

	UL_ASSIGN_BSWAP_HALF
	UL_ASSIGN_BSWAP_WORD
	UL_ASSIGN_BSWAP_XWORD

If you are reading/writing to relocation data, the following
routines understand relocation records and will get/set the
proper amount of data while handling any needed swapping:

	ld_reloc_targval_get()
	ld_reloc_targval_set()

Byte swapping is a fertile area for mistakes. If you're having trouble
getting a successful link in a cross link situation, you should always
do the experiment of doing the link on a platform with the same byte
order as the target. If that link succeeds, then you are looking at
a bug involving incorrect byte swapping.

-----------------------------------------------------------------------------
   As mentioned above, incorrect byte swapping is a common
error when developing libld target specific code. In addition to
trying a build machine with the same byte order as the target, elfdump
can also be a powerful tool for debugging. The first step with
elfdump is to simply dump everything and read it looking for obviously
bad information:

	% elfdump outobj 2>&1 | more

elfdump tries to do sanity checking on the objects it
displays. Hence, the following command is a a common
idiom:

	% elfdump outobj > /dev/null

Any problems with the file that elfdump can detect will be
written to stderr.

-----------------------------------------------------------------------------
Once you have the target-specific code in place, you must modify the
libld initialization code so that it will know how to use it. This
logic is found in

	usr/src/cmd/sgs/libld/common/ldmain.c

in the function ld_init_target().

-----------------------------------------------------------------------------
The ld front end program that uses libld must be modified so that
the "-z target=platform" command line option recognizes your
new target. This code is found in

	usr/src/cmd/sgs/ld/common

The change consists of adding an additional strcasecmp() to the
command line processing for -ztarget.

-----------------------------------------------------------------------------
You probably changed things getting your target integrated.
Please update this document to reflect your changes.