[Qemu-devel] Possible?: statically converting i386 ELF object files to ARM ELF object files

[Qemu-devel] Possible?: statically converting i386 ELF object files to ARM ELF object files

[Sebastian Nowozin]

Hello everybody,

(please quote my email address on a reply to this email, I am not member 
of the list, thanks.)

I have wondered wether the code conversion ability of qemu could be 
utilized to convert ELF .o object files compiled for i386 Linux by GCC 
to .o object files for ARM/Linux.  Is this possible or has something 
been attempted in this direction?

The need for such conversion occurs when porting the Java MIDP to ARM. 
Due to export restrictions, for some cryptography code in the MIDP, only 
linkable object files for i386 are available with the sources.  (Half a 
dozen small object files, clearly named like MD2.o MD5.o SHA1.o, ...).


Thanks in advance,
Sebastian

Re: [Qemu-devel] Possible?: statically converting i386 ELF object files to ARM ELF object files

[Paul Brook]

On Tuesday 19 April 2005 10:10, Sebastian Nowozin wrote:
> Hello everybody,
>
> (please quote my email address on a reply to this email, I am not member
> of the list, thanks.)
>
> I have wondered wether the code conversion ability of qemu could be
> utilized to convert ELF .o object files compiled for i386 Linux by GCC
> to .o object files for ARM/Linux.  Is this possible or has something
> been attempted in this direction?
>
> The need for such conversion occurs when porting the Java MIDP to ARM.
> Due to export restrictions, for some cryptography code in the MIDP, only
> linkable object files for i386 are available with the sources.  (Half a
> dozen small object files, clearly named like MD2.o MD5.o SHA1.o, ...).

The short answer is no,it's not possible.

In many cases it is possible to statically convert whole binaries from one 
arch to annother (http://www.complang.tuwien.ac.at/schani/bintrans/).

However in general it is not possible to convert a single object file/library 
then link it with native code. This would require inserting thunks on all the 
calls between native and translated code. To do this you need the full API 
used by the code. ie. what arguments are passed to each function, and how 
these arguments are used. C header files are not sufficient for this purpose 
by themselves.

To call non-native functions you need to use something like libffi 
(http://sources.redhat.com/libffi/), possibly with descriptions autogenerated 
from source annotations.

It would be possible to manually write thunks for a particular interface, but 
it's a lot of work, and has to be redone for every interface.

Paul

Re: [Qemu-devel] Possible?: statically converting i386 ELF object files to ARM ELF object files

[John Reiser]

Paul Brook wrote:
> On Tuesday 19 April 2005 10:10, Sebastian Nowozin wrote:
   [snip]
>>I have wondered wether the code conversion ability of qemu could be
>>utilized to convert ELF .o object files compiled for i386 Linux by GCC
>>to .o object files for ARM/Linux.
   [snip]
>>(Half a dozen small object files, clearly named like MD2.o MD5.o SHA1.o, ...).


> The short answer is no,it's not possible.
   [snip]
> It would be possible to manually write thunks for a particular interface, but 
> it's a lot of work, and has to be redone for every interface.


The even shorter answer is:  Just do it.

For this particular library, with 6 to 10 particular _leaf_ (!!) functions
(functions that don't call any other functions) whose calling sequence
can be guessed, the translation is easy.  Once you get to where qemu
takes care of the tedious part (register and opcode mapping), the rest
(inserting the relocation directives for access to a few static 'const'
arrays) is easy.

-- 
John Reiser, jreiser@BitWagon.com

Re: [Qemu-devel] Possible?: statically converting i386 ELF object files to ARM ELF object files

[Bob Deblier]

On Tue, 2005-04-19 at 17:10 +0800, Sebastian Nowozin wrote:
> Hello everybody,
> 
> (please quote my email address on a reply to this email, I am not member 
> of the list, thanks.)
> 
> I have wondered wether the code conversion ability of qemu could be 
> utilized to convert ELF .o object files compiled for i386 Linux by GCC 
> to .o object files for ARM/Linux.  Is this possible or has something 
> been attempted in this direction?
> 
> The need for such conversion occurs when porting the Java MIDP to ARM. 
> Due to export restrictions, for some cryptography code in the MIDP, only 
> linkable object files for i386 are available with the sources.  (Half a 
> dozen small object files, clearly named like MD2.o MD5.o SHA1.o, ...).

Wouldn't it be easier to write your own crypto functions conform to the
API used by the platform? All the algorithms you mention above are well
documented.

Sincerely,

Bob Deblier