[PATCH v3] riscv: cif: reduce shadow stack size limit from 4GB to 2GB

[David Laight]

On Fri, 15 May 2026 11:42:45 +0800
Zong Li <zong.li at sifive.com> wrote:

> On Thu, May 14, 2026 at 4:56 PM David Laight
..
> > I also don't understand the rational for just /2 and the 2G upper limit.
> > You need 512 nested function calls to even use 4k.
> > That would have to be quite deep recursion.  
> 
> During the discussions about the ARM GCS v3 series, community pointed
> out that a 4G shadow stack might be too large. This size is hard to
> support in memory-constrained environments like Android. However, the
> size cannot be too small either, or we might face stack overflow
> issues. At that time, a perfect size was not decided.

It is only VA not real memory so shouldn't make much difference to memory
use (except for nommu where the actual memory has to be allocated).

But 32bit programs with lots of threads can run out of VA.
Increasing the stack VA size by 50% might even give problems for 64bit
programs - if they are already reducing the thread stack size avoid
running out of VA.

I've not checked, but pthread_attr_setstacksize() sets a limit for the
thread stack size (which would otherwise default so rlimit(STACK)).
I don't believe it should update the rlimit value itself.
In which case you are using the wrong size.

But for a thread with a very reduced stack (say 128k) you probably only
need 1 page of shadow stack, any more could easily lead to running out
of VA.

-- David