Re: [PATCH RFT v17 4/8] fork: Add shadow stack support to clone3()

[Yury Khrustalev <yury.khrustalev@arm.com>]

On Mon, Jun 09, 2025 at 01:54:05PM +0100, Mark Brown wrote:
> Unlike with the normal stack there is no API for configuring the shadow
> stack for a new thread, instead the kernel will dynamically allocate a
> new shadow stack with the same size as the normal stack. This appears to
> be due to the shadow stack series having been in development since
> before the more extensible clone3() was added rather than anything more
> deliberate.
> 
> Add a parameter to clone3() specifying the shadow stack pointer to use
> for the new thread, this is inconsistent with the way we specify the
> normal stack but during review concerns were expressed about having to
> identify where the shadow stack pointer should be placed especially in
> cases where the shadow stack has been previously active.  If no shadow
> stack is specified then the existing implicit allocation behaviour is
> maintained.
> 
> If a shadow stack pointer is specified then it is required to have an
> architecture defined token placed on the stack, this will be consumed by
> the new task.  If no valid token is present then this will be reported
> with -EINVAL.  This token prevents new threads being created pointing at
> the shadow stack of an existing running thread.
> 
> If the architecture does not support shadow stacks the shadow stack
> pointer must be not be specified, architectures that do support the
> feature are expected to enforce the same requirement on individual
> systems that lack shadow stack support.
> 
> Update the existing arm64 and x86 implementations to pay attention to
> the newly added arguments, in order to maintain compatibility we use the
> existing behaviour if no shadow stack is specified. Since we are now
> using more fields from the kernel_clone_args we pass that into the
> shadow stack code rather than individual fields.
> 
> Portions of the x86 architecture code were written by Rick Edgecombe.
> 
> Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>

LGTM. I've tested this change on the FVP model along with my Glibc patch
series [1] and confirm that it works fine. The Glibc patch is at RFC stage
and will require more work, but it's enough to test this series.

Tested-by: Yury Khrustalev <yury.khrustalev@arm.com>

[1]: https://inbox.sourceware.org/libc-alpha/20250610151320.885131-1-yury.khrustalev@arm.com/

Thanks,
Yury