ld's targets vs emulations (intending to link EFI binaries on Linux)

Jan Beulich JBeulich@novell.com
Wed May 11 06:59:00 GMT 2011


Hi Nick,

>>> On 10.05.11 at 18:28, Nick Clifton <nickc@redhat.com> wrote:
>> What is the point of ld supporting (in e.g. the default x86 Linux
>> configurations) PE executables as targets, but not as emulations?
>> This basically means that one can link such executables, but there's
>> no control over the various linking parameters (as those are
>> processed by the respective [not present] emulation code).
> 
> Essentially this is because it is easier this way.  Supporting multiple, 
> dissimilar emulations would make the linker a lot more complex.

Reads sort of contradictory to me: The target (and hence the
output format) *is* supported, its the various special things that
can't be controlled.

Also things being easier isn't a really good reason for a partial
implementation imo.

> Changing the output format is actually a problematic feature of the 
> linker, and one that is disabled for some architectures.  It is much 
> cleaner to link without changing format and then use objcopy to convert 
> the resulting binary.  You are correct however in saying that objcopy 
> will not convert relocations and this is actually one of the big 
> problems with format conversions - there is rarely a good mapping 
> between the relocations of the formats.

Without any special relocations involved, there ought to be a pretty
clean mapping between ELF and PE relocations (on x86 at least).

Anyway, while for i386 to build a linker that can build both ELF
and full-featured PE, all it takes is an extra configure option
(--enable-target=i386-pe or some such), for x86-64 to be able
to do the same one needs to first introduce such a bfd and
linker target. Would a change like the below be acceptable for
mainline?

--- binutils-2.21/bfd/config.bfd
+++ 2.21/bfd/config.bfd
@@ -632,7 +632,7 @@ case "${targ}" in
     targ_selvecs="bfd_elf32_i386_vec i386linux_vec i386pei_vec x86_64pei_vec bfd_elf64_l1om_vec"
     want64=true
     ;;
-  x86_64-*-mingw*)
+  x86_64-*-mingw* | x86_64-*-pe | x86_64-*-pep )
     targ_defvec=x86_64pe_vec
     targ_selvecs="x86_64pe_vec x86_64pei_vec bfd_elf64_x86_64_vec bfd_elf64_l1om_vec i386pe_vec i386pei_vec bfd_elf32_i386_vec"
     want64=true
--- binutils-2.21/ld/configure.tgt
+++ 2.21/ld/configure.tgt
@@ -274,6 +274,9 @@ i[3-7]86-*-cygwin*)	targ_emul=i386pe ;
 			test "$targ" != "$host" && LIB_PATH='${tooldir}/lib/w32api' ;;
 i[3-7]86-*-mingw32*)	targ_emul=i386pe ;
 			targ_extra_ofiles="deffilep.o pe-dll.o" ;;
+x86_64-*-pe | x86_64-*-pep) targ_emul=i386pep ;
+			targ_extra_emuls=i386pe ;
+			targ_extra_ofiles="deffilep.o pep-dll.o pe-dll.o" ;;
 x86_64-*-mingw*)	targ_emul=i386pep ;
 			targ_extra_emuls=i386pe
 			targ_extra_ofiles="deffilep.o pep-dll.o pe-dll.o" ;;

(In the ld part I followed the model of not merging distinct entries,
albeit it would be possible to simply extend the mingw case - i386
has winnt, pe, and mingw all listed separately despite them all
specifying exactly the same.)

What the original question boils down to is whether i386 and x86_64
Linux selections shouldn't default to enable PE emulations (and not
just the respective BFD targets).

Thanks, Jan



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