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Date: Fri, 16 Oct 2015 11:31:31 +0700
From: Рысь <lynx@...xlynx.tk>
To: musl@...ts.openwall.com
Subject: Re: handling dlopen("/.../libc.so", ...) etc.
On Fri, 16 Oct 2015 00:18:32 -0400
Rich Felker <dalias@...c.org> wrote:
> On Fri, Oct 16, 2015 at 11:00:11AM +0700, Рысь wrote:
> > On Thu, 15 Oct 2015 23:46:42 -0400
> > Rich Felker <dalias@...c.org> wrote:
> >
> > > Presently, attempting to load "libc.so" (without pathname) or a
> > > number of other standard library names via dlopen suppresses the
> > > actual loading and returns a reference to the existing libc dso
> > > object. However, loading it via a pathname or alternate
> > > name/symlink will actually cause another copy to be loaded into
> > > memory (since we can't check the dev/ino against the existing
> > > one, because the kernel didn't give those to us) and bad things
> > > will happen. I've been thinking some about ways to prevent that.
> > >
> > > The most obvious way is to link libc.so with -Wl,-z,nodlopen and
> > > make the dynamic linker enforce DF_1_NOOPEN, but this would cause
> > > the load to fail when we probably want it to succeed but return a
> > > reference to the existing libc.
> > >
> > > Another option would be to somehow encode musl's identity in
> > > libc.so so that the loader code can check "is the dso we've just
> > > loaded actually musl?" In that case it can abort the load and use
> > > the existing libc instead. Options for how to do this might
> > > include a special reserved-namespace symbol. If an approach like
> > > this is taken, it would be ideal to be able to detect
> > > existing/previous versions of libc.so (to avoid loading them
> > > too), and the approach should be future-proof so that the current
> > > libc.so can avoid loading future versions of itself, and so that
> > > future versions can avoid loading the current version.
> > >
> > > I'd like to hear any further ideas on how to achieve this.
> >
> > Who would even want to load "libc.so"? I mean, does not it already
> > being loaded in every libc implementations with dynamic linking
> > support already today?
> >
> > And what are use cases? Who does this today? I am confused.
>
> It could be a program calling dlopen on the pathname of each library
> it knows is loaded (e.g. obtained from dl_iterate_phdr or
> /proc/self/maps) to obtain handles for them -- the intent then is
> _not_ to load anything new, but to get module handles for stuff that's
> already loaded. In that case RTLD_NOLOAD should be used, but it might
> not be, and even if it is used you want success rather than failure.
> Similar situations could arise from programmatically reading DT_NEEDED
> records to manually load dependencies before loading a library, e.g.
> to try to control order ctors run or something.
>
> In any case, the main concern is to _prevent_ wrongful loading of
> multiple copies of libc.so that could read to serious issues like
> memory corruption. The -Wl,-z,nodlopen approach solves this, but it
> would be nicer if attempting to open a full pathname to libc.so
> behaved just like opening the string "libc.so".
>
> Does that answer your questions?
>
> Rich
>
OK. You have a good ELF interpreter inside - just add object file "tag"
and match it when you're reading ELF structures from file.
--
http://lynxlynx.tk/
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