[PATCH v2 2/3] nptl: Add libc allocated shadow stack for new threads

[Mark Brown]

On Thu, Sep 18, 2025 at 03:33:57PM -0700, H.J. Lu wrote:
> On Thu, Sep 18, 2025 at 3:23 PM Mark Brown <broonie@kernel.org> wrote:

> > It is currently based on RLIMIT_STACK.

> Can you point me to the kernel code where shadow stack size is determined?

x86 code is here: 

   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/shstk.c#n225

(it's the also using the normal stack size as well as RLIMIT_STACK).

> > > 2.  Can it be solved differently with a prctl call?

> > Yes, that has been suggested (including earlier in this thread) to try
> > to help users who are restricted to regular clone() - either configuring
> > the ratio or an absolute number.  There's been limited enthusiasm for it
> > as the only option due to clunkiness if userspace wants different values
> > for different threads (though that would be the common case of course).

> The shadow stack configure option will be active until the next prctl call.
> In the meantime, clone will use the same shadow stack configure option.
> Different threads can have different shadow stack sizes.

Right, it wasn't a "this doesn't work" but rather a "this is clunky,
obviously we should be able to do this as part of clone3() instead"
thing when it's come up previously.
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