On Wed, Feb 16, 2022 at 01:34:25PM +0000, Catalin Marinas wrote:
> On Tue, Feb 15, 2022 at 06:34:56PM +0000, Will Deacon wrote:

> > This appears to be a user-visible change which cannot be detected or
> > disabled from userspace. If there is code out there which does not work
> > when BTI is enabled, won't that now explode when the kernel enables it?
> > How are we supposed to handle such a regression?

> If this ever happens, the only workaround is to disable BTI on the
> kernel command line. If we need a knob closer to user, we could add a
> sysctl option (as we did for the tagged address ABI, though I doubt
> people are even aware that exists). The dynamic loader doesn't do
> anything smart when deciding to map objects with PROT_BTI (like env
> variables), it simply relies on the ELF information.

The dynamic loader is the place where I'd expect to do any
per-executable workarounds, but currently that's not actually
implemented anywhere.  Someone could also make a tool to strip BTI
annotations from executables.

> I think the only difference would be with a BTI-unware dynamic loader
> (e.g. older distro). Here the main executable, if compiled with BTI,
> would be mapped as executable while the rest of the libraries are
> non-BTI. The interworking should be fine but we can't test everything
> since such BTI binaries would not normally be part of the distro.

> If there are dodgy libraries out there that do tricks and branch into
> the middle of a function in the main executable, they will fail with
> this series but also fail if MDWE is disabled and the dynamic linker is
> BTI-aware. So this hardly counts as a use-case.

I'm not aware of any issues we've run into which are due to interworking
between binaries rather than within a binary due to either miscompilation
or doing something in hand coded assembler that needs updating for BTI.
It doesn't mean it can't happen but it's hard to see what people might
be doing.

> For consistency, I think whoever does the initial mapping should also
> set the correct attributes as we do for static binaries. If you think
> another knob is needed other than the cmdline, I'm fine with it.

Might also be worth pointing out that we already map the vDSO with BTI
enabled if it's built with BTI.