Re: dynamic linking (Re: [musl] musl and android)

[Rich Felker <dalias@libc.org>]

On Thu, Jan 15, 2015 at 02:04:32PM +0100, u-wsnj@aetey.se wrote:
> On Thu, Jan 15, 2015 at 07:15:36AM -0500, Rich Felker wrote:
> > > > and executing the program via a wrapper script that manually invokes
> > > > the dynamic linker (so the hard-coded PT_INTERP pathname isn't
> > > > needed).
> 
> > > Actually I believe (and know from long time experience) this to be
> > > the only "sane"/robust/general way to run dynamically linked executables.
> > 
> > It depends on your perspective. If you're viewing them as
> > self-contained entities, then yes, but if you're viewing them as
> > something running in an existing library ecosystem, there's no
> > problem.
> 
> Right, it depends. For the second perspective you seem to imply that
> an "ecosystem" is to be managed in a certain way.
> 
> Binaries belonging to my "library ecosystems" still can be subject to
> the C library tests and upgrades on a per-binary basis, not only "all
> binaries belonging to the same ecosystem at once", which is of course
> possible as well.
> 
> This would be impossible if I'd rely on the hardcoded loader path.

That's why I said i depends on your perspective. From your perspective
this does not work. From most traditional distributions' perspectives,
it does.

> > > I don't think that the implications of hardcoding the interpreter
> > > path were well understood when dynamic linking was first deployed,
> > > the hardcoding merely became percepted as the only/natural approach
> > > when the purpose was to cheaply imitate the behaviour of statically
> > > linked programs. (This mimics the #!/... which is similarly
> > > limited/broken. The plain text scripts are though relatively easy
> > > to modify to hack around the limitation, according to local curcumstances)
> > 
> > I think this could be fixed easily by having the kernel support
> > $ORIGIN in PT_INTERP.
> 
> Unfortunately, no. $ORIGIN does not and can not replace a run time
> choice of the dynamic loader. As a simple example, consider a binary on
> a readonly media. How would you convince the kernel to run a different
> loader than assumed (among others) by the path to the mount point of
> the media? In my eyes the mounting of the media (possibly with lots of
> binaries on it) and running a certain loader for a certain binary are
> very different things and do not have to / should not depend on each other.

If you're distributing the binary and dynamic loader/libc and all
libraries it needs together, I'd assume they'd all be in the same
directory, or else in ../lib/ relative to the binary. In that case
$ORIGIN works perfectly fine. Note that $ORIGIN is _not_ an
environment variable; it's a dynamic-linker feature for locating
libraries relative to the ELF file (main executable or other library)
that needs (via DT_NEEDED) them, and the same concept would work for
PT_INTERP.

Rich