Re: RFC: adding Linux vsyscall-disable and similar backwards-incompatibility flags to ELF headers?

From: Andy Lutomirski
Date: Tue Sep 01 2015 - 22:24:03 EST

On Sep 1, 2015 6:12 PM, "Ian Lance Taylor" <iant@xxxxxxxxxx> wrote:
> On Tue, Sep 1, 2015 at 5:51 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >
> > Linux has a handful of weird features that are only supported for
> > backwards compatibility. The big one is the x86_64 vsyscall page, but
> > uselib probably belongs on the list, too, and we might end up with
> > more at some point.
> >
> > I'd like to add a way that new programs can turn these features off.
> > In particular, I want the vsyscall page to be completely gone from the
> > perspective of any new enough program. This is straightforward if we
> > add a system call to ask for the vsyscall page to be disabled, but I'm
> > wondering if we can come up with a non-syscall way to do it.
> >
> > I think that the ideal behavior would be that anything linked against
> > a sufficiently new libc would be detected, but I don't see a good way
> > to do that using existing toolchain features.
> >
> > Ideas? We could add a new phdr for this, but then we'd need to play
> > linker script games, and I'm not sure that could be done in a clean,
> > extensible way.
> What sets up the vsyscall page, and what information does it have
> before doing so?
> I'm guessing it's the kernel that sets it up, and that all it can see
> at that point is the program headers.

Currently it's global and nothing thinks about it per-process at all.
The kernel can do whatever it likes going forward, subject to
backwards compatibility. Doing something at ELF load time is probably
the right approach.

> We could pass information using an appropriate note section. My
> recollection is that the linkers will turn an SHF_ALLOC note section
> into a PT_NOTE program header.

Oh, interesting. I'll check that. Glibc and competitors could add
notes to their statically-linked bits.

The unpleasant case is a new dynamic binary linked against an old
libc, but that might be irrelevant in practice. After all, I think
that a lot of libc competitors never supported the vsyscall page at
all, and even glibc isn't really backwards compatible that way.

We could also require that both the binary and interpreter have the
note, which would more or less solve the backwards compatibility

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