Date: Wed, 7 Apr 1999 15:38:21 +0930 From: Greg Lehey <grog@lemis.com> To: Peter Wemm <peter@netplex.com.au> Cc: Archie Cobbs <archie@whistle.com>, Christopher Michaels <ChrisMic@clientlogic.com>, gjb@comkey.com.au, questions@FreeBSD.ORG, hackers@FreeBSD.ORG Subject: Re: Debug kernel by default (was: System size with -g) Message-ID: <19990407153821.L2142@lemis.com> In-Reply-To: <199904070557.NAA03953@spinner.netplex.com.au>; from Peter Wemm on Wed, Apr 07, 1999 at 01:57:56PM %2B0800 References: <19990407082416.H2142@lemis.com> <199904070557.NAA03953@spinner.netplex.com.au>
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On Wednesday, 7 April 1999 at 13:57:56 +0800, Peter Wemm wrote:
> Greg Lehey wrote:
> [..]
>> A number of other people have observed that the current boot loader
>> doesn't load the symbols anyway, so you could install /kernel.debug
>> only and still run normally. I consider this a deficiency in the boot
>> loader, since it should be possible to load the symbols. Of course,
>> an alternative solution would be to install /kernel.debug and leave it
>> until boot time to decide whether to load the symbols. That would
>> have the great advantage that you wouldn't get any mismatch between
>> the two kernels.
>
> I'll chime in here since this was the result of something I spent a lot
> of time on.
>
> A couple of points:
> 1: The loader *does* load symbols, just not the debugging ones on elf. It
> does this for runtime linking purposes.
Good point. I suppose that for the benefit of others I should point
out that current kernels always (should) contain some symbols (see the
reference to ps(1) in another message I sent. We're talking about
full ("debug") symbols here.
> 2: Under ELF, the debug symbols are *seperate* to the symbol table.
Aren't they under a.out as well? Otherwise strip(1) would have a lot
of trouble stripping. That's probably why the bootstrap doesn't load
them, too.
> 3: A number of compromises had to be made to get around various
> restrictions in the environment being worked in.
>
> At present, the only symbols that DDB *might* be able to use from the debug
> symbols in the .stabs section are the line number tables to associate an
> address with a line number of source. This is an awful lot of baggage to
> carry for such a small gain. Yes, it would be nice to have a traceback show
> line numbers in the list, but I doubt it's worth implementation cost since
> the addresses can be matched in other ways.
Good point.
> On the whole though, I feel that the best way to deal with loading
> extra debugging symbols is to do it shortly after boot.
Before or after starting process 0?
> And on the subject of debugging kernels getting built, I'd tend to
> agree. Don't install them though, install the stripped version.
I had planned to leave that to the user: 'make install' will install a
stripped kernel, 'make install.debug' will install the full symbol
kernel. I still think this is a reasonable compromise.
> Incidently, the loader works on kernels that have been plain
> stripped too, but it's restricted to global symbols only as a
> result.
I'm not sure I understand this. By "plain stripped" do you mean strip
-g or strip -s? The way I see it, the result of 'strip -g' is
identical to a kernel built without the -g option to config.
Greg
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