Re: [PATCH 1/3] Add fbsd_nat_add_target.

[John Baldwin <jhb at freebsd dot org>]

On Monday, April 27, 2015 09:54:24 PM Mark Kettenis wrote:
> > From: John Baldwin <jhb@freebsd.org>
> > Date: Mon, 27 Apr 2015 15:16:50 -0400
> > 
> > On Monday, April 27, 2015 08:10:18 PM Pedro Alves wrote:
> > > On 04/26/2015 02:24 AM, John Baldwin wrote:
> > > > Add a wrapper for add_target in fbsd-nat.c to override target operations
> > > > common to all native FreeBSD targets.
> > > > 
> > > > gdb/ChangeLog:
> > > > 
> > > > 	* fbsd-nat.c (fbsd_pid_to_exec_file): Mark static.
> > > > 	(fbsd_find_memory_regions): Mark static.
> > > > 	(fbsd_nat_add_target): New function.
> > > > 	* fbsd-nat.h: Export fbsd_nat_add_target and remove prototypes for
> > > > 	fbsd_pid_to_exec_file and fbsd_find_memory_regions.
> > > > 	* amd64fbsd-nat.c (_initialize_amd64fbsd_nat): Use fbsd_nat_add_target.
> > > > 	* i386fbsd-nat.c (_initialize_i386fbsd_nat): Likewise.
> > > > 	* ppcfbsd-nat.c (_initialize_ppcfbsd_nat): Likewise.
> > > > 	* sparc64fbsd-nat.c (_initialize_sparc64fbsd_nat): Likewise.
> > > 
> > > OOC, any reason you didn't instead do it like:
> > > 
> > > struct target_ops *
> > > fbsd_nat_target (void)
> > > {
> > >   struct target_ops *t = inf_ptrace_target ();
> > > 
> > >   t->to_pid_to_exec_file = fbsd_pid_to_exec_file;
> > >   t->to_find_memory_regions = fbsd_find_memory_regions;
> > >   return t;
> > > }
> > > 
> > > and then use fbsd_nat_target instead of inf_ptrace_target
> > > directly?
> > > 
> > > This maps a little better to a C++ world.
> > > 
> > > linux-nat.c does it the way you did as it keeps a separate
> > > linux_ops target instance around.
> > 
> > I was probably just using linux-nat.c as a reference.  One thing that
> > confuses me about the linux-nat target is that it keeps linux_ops
> > around so that it can call the original methods that it overrides,
> > and yet for a few methods it also uses a local 'super_foo' variable
> > to call an original method.  I think that those are both doing the
> > same thing, but perhaps there is some subtlety I'm missing?
> > 
> > I do use a 'super_wait' to call ptrace's wait method in the second
> > patch in this series, so I could certainly change this to return a
> > target rather than modifying an existing one if that is preferred.
> 
> I'd say the linux-nat.c code is a bad example and recommend looking at
> the obsd-nat.c code instead.  The linux-nat.c code is so complicated
> because of all the workarounds needed to support threads.  The
> linux_ops stuff is pretty much an artifact of those workarounds.  
> 
> I found that to add threads-support I did need to make modifications
> to the _wait function that made it hard to re-use the
> inf_ptrace_wait() code.  Sometimes code duplications just is the right
> answer.

FWIW, obsd-nat.c (which I have looked at a bit), also uses a wrapper
around add_target rather than creating a generic OpenBSD native target.
To change the FreeBSD native targets I think would be an invasive change
since they use platform-specific native targets that are pan-BSD as their
initial target (e.g. amd64bsd_target) and customize from there.  To make
fbsd_nat_target work I would need to rework things like amd64bsd_target
to modify an existing target instead of returning a new one I think (which
would also mean changing all the other BSD native targets).

-- 
John Baldwin