Re: [RFC] Boehm GC changes for uClibc.

[David Daney <ddaney at avtrex dot com>]

Joakim Tjernlund wrote:
>  
>
>
> > -----Original Message-----
> > From: uclibc-bounces@uclibc.org 
> > [mailto:uclibc-bounces@uclibc.org] On Behalf Of David Daney
> > Sent: den 17 april 2006 20:39
> > To: Java Patch List
> > Cc: uclibc@uclibc.org; Hans Boehm
> > Subject: [RFC] Boehm GC changes for uClibc.
> >
> > I am trying to get libgcj running under uClibc.  This patch 
> > is necessary
> >  to get a simple 'Hello World' program to run.  Other 
> > patches will be needed for full functionality, but this one 
> > is a good start.
> >
> > The problem is the the Boehm GC is expecting to be able to 
> > find the link_map via the DT_DEBUG dynamic tag.  The uClibc 
> > dynamic loader does not initialize DT_DEBUG (at least for 
> > MIPS) because doing so would dirty the page where it resides.
>
>
> Yes, MIPS is the only arch not setting DT_DEBUG. MIPS uses
> DT_MIPS_RLD_MAP instead. Look at dl-sysdep.h

I will investigate doing that instead.

> BTW, 
> Whats the problem if MIPS also sets DT_DEBUG?

As I said before, it dirties the page where DT_DEBUG resides.  The 
DYNAMIC section lays within a read-only LOAD section.  Modifying it 
requires that you temporarily make it writable, modify the DT_DEBUG tag, 
and then make it read-only.  This dirties a page.  There was a several 
month period a couple of years ago when uClibc did that.  Evidently it 
was deemed a bad thing.

> > The patch gains access the link_map via the global symbol 
> > _dl_debug_addr.  Without the patch there is a SIGSEGV 
> > dereferencing the zero valued DT_DEBUG tag while doing 
> > _Jv_CreateJavaVM().
>
>
> _dl_debug_addr is misnamed in uClibc. It should be named _r_debug instead.

Yes, I discovered that too.

> On top of that I can't remember if _r_debug is a glibc thing or if
> it is a "standard" symbol. If it is a glibc symbol it might be removed at some point.
> If you must use _dl_debug_addr then please rename it and make if work as
> in glibc first.

Unfortunately I need to make it work with uClibc-0.9.27, so a 
retroactive change is not possible.  In the interests of making the 
changes to Boehm GC as small and clean as possible, I want to stay away 
from hacking up the uClibc trunk and having to support two different 
ways of doing it.

David Daney.