Re: [PATCH] Workaround a FreeBSD ptrace() bug with clearing thread events.

[John Baldwin <jhb at freebsd dot org>]

On Friday, March 02, 2018 07:31:49 PM Maciej W. Rozycki wrote:
> On Fri, 2 Mar 2018, John Baldwin wrote:
> 
> > >  Hmm, doesn't it have to be a run-time check then?  Otherwise you're 
> > > basing your decision on the host system GDB has been built for and not one 
> > > it will be run on, which I suppose does not necessarily have to be of the 
> > > same version.  Or am I missing anything here?
> > 
> > FreeBSD generally does not support forwards-compatability for binaries (newer
> > binary on older kernel), only backwards-compatability (older binary on newer
> > kernel).  In this case, using the workaround is also fine on a fixed kernel,
> > so it doesn't hurt if GDB is compiled on an older system (thus using the
> > workaround) and then run under a newer kernel.
> 
>  Fair enough.
> 
>  Is the one-way compatibility enforced though, by a system library runtime 
> or the kernel somehow, by refusing to run a binary built for a kernel that 
> is newer than one currently in charge of the system?  Otherwise the rule 
> would be quite fragile and error prone, asking for extra care to be taken 
> by the user.

It is enforced in some ways but not others.  Kernel modules do depend on a
version number in such a way that attempting to load a newer kernel module
on an older kernel will fail.  However, the general policy of only supporting
one-way compatibility is well-known among the FreeBSD userbase (for example,
the instructions for upgrading a system from source require booting into a
new kernel before installing the matching userland binaries).

-- 
John Baldwin