[PATCH v6 16/33] riscv/shstk: If needed allocate a new shadow stack on clone

[Edgecombe, Rick P]

On Tue, 2024-10-08 at 16:17 -0700, Deepak Gupta wrote:
> Yeah you're right. Honestly, I've been shameless in adapting most of the flows
> from x86 `shstk.c` for risc-v. So thank you for that.

All good, glad we ended up with similar behavior.

> 
> Now that we've `ARCH_HAS_USER_SHADOW_STACK` part of multiple patch series (riscv
> shadowstack, clone3 and I think arm64 gcs series as well). It's probably the
> appropriate time to find common grounds.

There have been bugs in the similar bits of code. So will be nice to not have to
fix them in each arch too.

> 
> This is what I suggest
> 
> - move most of the common/arch agnostic shadow stack stuff in kernel/shstk.c
>    This gets part of compile if `ARCH_HAS_USER_SHADOW_STACK` is enabled/selected.

Yea, I guess we have commonality for (in x86 naming):
 - map_shadow_stack()
 - shstk_free()
 - shstk_alloc_thread_stack()
 - shstk_setup()

The signal part starts to diverge. Then I guess x86 has a different prctl
interface.

> 
> - allow arch specific branch out guard checks for "if cpu supports", "is shadow stack
>    enabled on the task_struct" (I expect each arch layout of task_struct will be
>    different, no point finding common ground there), etc.

Sure.

> 
> I think it's worth a try. 
> If you already don't have patches, I'll spend some time to see what it takes to
> converge in my next version. If I end up into some roadblock, will use this thread
> for further discussion.

Sounds good. I have not looked at it too much.