On Thu, Nov 16, 2023 at 06:11:17PM +0000, Edgecombe, Rick P wrote:

Now that I've thought about it more, removing the CLONE_VFORK part of
the logic has several downsides. It is a little extra work to create
and unmap a shadow stack for an operation that is supposed to be this
limited fast thing.

It does rather feel like it's defeating the point of the thing.

It also will change the SSP(let me know if anyone has a general term we
can use) for the child. So if you have like:

SSP seems fine, we're already using shadow stack here.

What about a CLONE_NEW_SHSTK for clone3 that forces a new shadow stack?
So keep the existing logic, but the new flag can override the logic for
!CLONE_VM and CLONE_VFORK if the caller wants. The behavior of
shadow_stack_size is then simple. 0 means use default size, !0 means
use the passed size. No need to overload and tie up args->stack.

That does seem like it cuts through the ambiguous cases.  If we go for
that it feels like we should require the flag when specifying a size,
just to be sure that everything is clear.  Though having said that we
could just always allocate a shadow stack if a size is specified
regardless of the flags, requiring people who want non-default behaviour
to have some idea what stack size they want.  I don't think I have
strong opinons between having the new flag or always allocating a stack
if a size is specified.