Date: Thu, 15 Jan 2015 08:34:45 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: dynamic linking (Re: musl and android) On Thu, Jan 15, 2015 at 02:04:32PM +0100, u-wsnj@...ey.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
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