Re: Question about _dl_debug_state and new glibc

[Mark Kettenis <mark.kettenis@xs4all.nl>]

   Date: Mon, 11 Oct 2004 13:55:25 -0400
   From: Daniel Jacobowitz <drow@false.org>

   On Mon, Oct 11, 2004 at 10:46:47AM -0700, Kevin Buettner wrote:
   > On Mon, 11 Oct 2004 10:22:07 -0700
   > Randolph Chung <randolph@tausq.org> wrote:
   > 
   > > I've been away from gdb for a bit, so hopefully this is not something
   > > simple I missed while I've been away :)
   > > 
   > > It appears that newer versions of glibc now marks the _dl_debug_state
   > > function as hidden, making it not visible to gdb. This breaks the
   > > solib tracking code in solib-svr4.c.....
   > > 
   > > Looking at the docs, it looks like the "proper" way for gdb to do this
   > > is to look up the r_debug symbol and use the r_brk member to locate 
   > > _dl_debug_state's address. is there any particular reason why we don't
   > > do this in gdb?

   Only for static executables usually; for dynamic executables it's
   supposed to be DT_DEBUG if that's available.  Same difference.

Hey wake up Daniel.  Static executables don't have shared libraries.

   > I can't think of any reason.  I'm guessing that it was more expedient
   > for the original authors of the code to do it the way it was done.
   > In any case, I know of no reason not to change it so that it's done
   > "properly".

   We set the breakpoint on _dl_debug_state before starting the inferior.
   _dl_debug_initialize initializes _r_debug somewhat later...  I'm not
   sure how it is supposed to be used.  Do either of you know?

The problem is that even DT_DEBUG isn't initialized before
initialization of the dynamic linker.  Theoretically, we could put a
watchpoint on DT_DEBUG, and lookup r_brk if it triggers.  However, I
don't think that's very practical on platforms without hardware
watchpoint support.

I suppose this is just something that has never been completely
implemented.  Somehow I think the dynamic linker was supposed to trap
just after initializing r_debug and setting DT_DEBUG if it was being
traced, perhaps if it noticed that DT_DEBUG was set to some special
value by the debugger.  Perhaps we'll know when "Open Solaris" is
released.

   I see it hasn't been exported since March.  Blech.

So we should ask the glibc developers to unhide _dl_debug_state.

Mark