Help with cross-compiling GDB

[Paul Smith <psmith@gnu.org>]

Hi all.  I'm trying to create a complete toolchain including GCC,
binutils, plus tools like make, fakeroot, bison, flex, m4, and of course
GDB.

What I need to do is have the tools compiled to be able to run on much
older GNU/Linux distributions, even though my build system is very
recent.  So I have a cross-compiler built that uses a sysroot from an
older distribution and I'm using that to compile GDB.  I need to be able
to compile and debug both 32bit and 64bit programs.  Originally I wanted
to have a 32bit version of GDB which could debug both, but that doesn't
appear to be possible.  That's OK, I'm willing to have GDB be a 64bit
application, or else I'll build GDB twice.  For now I'm just trying to
get the 64bit GDB working.

So, I configure GDB with something like this:

  ./configure --disable-nls --disable-werror --prefix=/tmp/invalid/generic \
    --host=x86_64-olddist-linux-gnu --target=x86_64-generic-linux-gnu \
    --with-sysroot=/tmp/invalid/sysroot/generic/sysroot --enable-64-bit-bfd \
    --enable-tui --with-expat --without-python \
    --program-prefix=x86_64-generic-linux-gnu-

So this builds fine, but when I try to debug something I always get
errors:

$ ./obj/cc/generic/bin/x86_64-generic-linux-gnu-gdb foo
GNU gdb (GDB) 7.5
    ...
This GDB was configured as "--host=x86_64-olddist-linux-gnu
--target=x86_64-generic-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from foo...done.

(gdb) run
Starting program: foo 
Don't know how to run.  Try "help target".

(gdb) show arch
The target architecture is set automatically (currently i386)
(gdb) set arch
Requires an argument. Valid arguments are i386, i386:x86-64, i386:x64-32, i8086,
i386:intel, i386:x86-64:intel, i386:x64-32:intel, auto.


This is a 32bit app but I get identical errors (although different show
arch of course) when I try to debug a 64bit app.  I can set breakpoints,
etc. but any attempt to run gives the "don't know how to run" error.

I'm not sure exactly what that means in this context.  It appears to
have the right architecture and that architecture appears to be
supported.  So what does GDB need from me?