Generating userland debugging symbols

[Mark Johnston]

On Fri, Oct 29, 2010 at 06:05:08PM -0700, Garrett Cooper wrote:
>     But having a strip script might be useful. Some companies brand
> binaries for their own purposes, so having a hook into a strip script
> (it should be no more than a few lines), should suffice. Something
> that my old group used was similar to what's described in the objcopy
> manpage:
> 
>            The  intention is that this option will be used in conjunction with
>            --add-gnu-debuglink  to  create  a  two  part  executable.   One  a
>            stripped  binary  which will occupy less space in RAM and in a dis-
>            tribution and the second a debugging information file which is only
>            needed  if  debugging abilities are required.  The suggested proce-
>            dure to create these files is as follows:
> 
>            1.<Link the executable as normal.  Assuming that is is called>
>                "foo" then...
> 
>            1.<Run "objcopy --only-keep-debug foo foo.dbg" to>
>                create a file containing the debugging info.
> 
>            1.<Run "objcopy --strip-debug foo" to create a>
>                stripped executable.
> 
>            1.<Run "objcopy --add-gnu-debuglink=foo.dbg foo">
>                to add a link to the debugging  info  into  the  stripped  exe-
>                cutable.
> 
>     That's probably similar to what your group is doing...
>     Of course a more generalized solution also might be smart if clang
> / llvm decides to go off in a completely different direction
> syntactically from binutils. Hence a custom tailored set of scripts
> may or may not be the way to go (is pmake's .USE directive another
> option?).

That's almost exactly what we're doing. I also think it's easier to go with a
script, but the required changes to strip(1) and install(1) are pretty
basic. strip(1) and objcopy(1) are both generated from the same source
file, so it's just a matter of adding an extra option that performs the
above operations in one command. I have no specific preference on this
point though.

I'm not familiar with pmake - I'll take a look into it.

> 
>     I agree with kib@ on both counts and I like this idea. Maybe the
> directory should be something like /usr/obj/stripped for the stripped
> binaries and /usr/obj/debug for the debug symbols? (just something I'm
> tossing out... the hierarchy could be better organized than that..).
> It might get a bit more hairy to clean up later, but oh well, it's for
> development :D...
> Thanks!
> -Garrett

It took me a while to figure this out, but the absolute path to the
debug symbols isn't encoded in the .gnu_debuglink segment - just the
filename is placed there. When gdb loads a binary, it searches a
predefined set of directories for the file recorded in .gnu_debuglink
and loads it if it finds it. One of the directories is global - on
FreeBSD it seems to be /usr/local/lib/debug/ by default. You can
configure it with

show debug-file-directory
and
set debug-file-directory

Thanks,
-Mark