Re: [PATCH] Remove incorrect special handling of /usr/libexec

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 26 Aug 2024 18:47:27 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Mon, Aug 26, 2024 at 10:37:23AM +0200, Stefano Brivio wrote:
> > On Mon, 26 Aug 2024 18:20:35 +1000
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Mon, Aug 26, 2024 at 09:55:47AM +0200, Stefano Brivio wrote:  
> > > > On Mon, 26 Aug 2024 16:39:01 +1000
> > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > >     
> > > > > The statement in the comment about /usr/libexec being only for running on
> > > > > other hosts simply isn't true, neither in practice nor according to the
> > > > > FHS spec[0].    
> > > > 
> > > > I don't remember where I took that meaning of /usr/libexec from, I
> > > > guess it's from some outdated packaging guidelines (Fedora? Kata
> > > > Containers?). Sure, it makes sense to fix that.
> > > >     
> > > > > Furthermore this logic didn't even handle it correctly, since
> > > > > it would only handle binaries _directly_ in /usr/libexec, not those in
> > > > > (explicitly FHS permitted) subdirectories under /usr/libexec.    
> > > > 
> > > > So, this change breaks the two cases I needed to cover with this, which
> > > > are /usr/libexec/kata-agent in general, and /usr/libexec/qemu-kvm on
> > > > RHEL 9.    
> > > 
> > > Huh.. why?  
> > 
> > Because they're not in PATH on the guest, so we can't execute them.  
> 
> But.. they wouldn't have been in the PATH on the host either, so
> whatever front end binary is using them must have found them by some
> other means.

If you actually use the front-end binary, sure. The issue is the
interpretation of "intended" in the FHS description:

  /usr/libexec includes internal binaries that are not intended to be
  executed directly by users or shell scripts.

...not intended on a specific distribution? Or due to their nature?

> > As an alternative, we can unconditionally add /usr/libexec to it using
> > $FIXUP. I added the lines moving stuff to /usr/bin before I implemented
> > the $FIXUP mechanism, and I needed to run kata-agent as init.
> > 
> > But now that $FIXUP is available, that's probably less invasive.
> >   
> > > > What does it fix?    
> > > 
> > > I don't have a concrete case, but it would break anything where we're
> > > including this support binary, but the "front end" binary looks for it
> > > explicitly in /usr/libexec.  Which I'd kind of expect to be most
> > > support binary cases, since by design /usr/libexec won't generally be
> > > in the PATH.  
> > 
> > I see. Well, given the limited time I can spend on maintaining mbuto,
> > I'd really prefer to just fix concrete issues, but this looks obvious
> > enough -- as long as we have another way to keep qemu-kvm usable in the
> > guest.  
> 
> Ah... I guess for qemu-kvm we're intentionally taking what's a support
> binary on the host and using it as a primary binary on the guest.

Right, same for kata-agent.

> That's different from the sshd-session case, where it's a support
> binary in both environments.
> 
> I'd favour leaving the path of the binary itself alone and explicitly
> adding a link from /usr/bin for the qemu-kvm case.

We could add a link from /usr/bin for all the paths we find in
/usr/libexec, then, to keep it more general. But is it really worth the
effort compared to just adding /usr/libexec to $PATH on the guest?

-- 
Stefano