[musl] Re: [PATCH v8 00/38] arm64/gcs: Provide support for GCS in userspace

[Mark Brown]

On Wed, Feb 21, 2024 at 12:57:19PM -0500, dalias at libc.org wrote:
> On Wed, Feb 21, 2024 at 05:36:12PM +0000, Mark Brown wrote:

> > This feels like it's getting complicated and I fear it may be an uphill
> > struggle to get such code merged, at least for arm64.  My instinct is
> > that it's going to be much more robust and generally tractable to let
> > things run to some suitable synchronisation point and then disable
> > there, but if we're going to do that then userspace can hopefully
> > arrange to do the disabling itself through the standard disable
> > interface anyway.  Presumably it'll want to notice things being disabled
> > at some point anyway?  TBH that's been how all the prior proposals for
> > process wide disable I've seen were done.

> If it's possible to disable per-thread rather than per-process, some
> things are easier. Disabling on account of using alt stacks only needs

Both x86 and arm64 currently track shadow stack enablement per thread,
not per process, so it's not just possible to do per thread it's the
only thing we're currently implementing.  I think the same is true for
RISC-V but I didn't look as closely at that yet.

> to be done on the threads using those stacks. However, for dlopen
> purposes you need a way to disable shadow stack for the whole process.
> Initially this is only needed for the thread that called dlopen, but
> it needs to have propagated to any thread that synchronizes with
> completion of the call to dlopen by the time that synchronization
> occurs, and since that synchronization can happen in lots of different
> ways that are purely userspace (thanks to futexes being userspace in
> the uncontended case), I don't see any way to make it work without
> extremely invasive, high-cost checks.

Yeah, it's not particularly nice - any whole process disable is going to
have some nasty cases I think.  Rick's message about covered AFAIR the
discussion, there were also some proposals for more limited userspaces I
think.

> If folks on the kernel side are not going to be amenable to doing the
> things that are easy for the kernel to make it work without breaking
> compatibility with existing interfaces, but that are impossible or
> near-impossible for userspace to do, this seems like a dead-end. And I
> suspect an operation to "disable shadow stack, but without making
> threads still in SS-critical sections crash" is going to be
> necessary..

Could you be more specific as to the easy things that you're referencing
here?
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