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

[Zong Li]

On Fri, Apr 24, 2026 at 5:26 PM David Laight
<david.laight.linux at gmail.com> wrote:
>
> On Thu, 23 Apr 2026 23:55:40 -0700
> Zong Li <zong.li at sifive.com> wrote:
>
> > Follow the ARM64 GCS (Guarded Control Stack) implementation approach
> > by reducing the shadow stack size allocation from min(RLIMIT_STACK, 4GB)
> > to min(RLIMIT_STACK/2, 2GB). see commit '506496bcbb42 "arm64/gcs: Ensure
> > that new threads have a GCS")'
> >
> > Rationale:
> >
> > 1. Shadow stacks only store return addresses (8 bytes per entry), not
> >    local variables, function parameters, or saved registers. A 2GB
> >    shadow stack is far more than sufficient for any practical
> >    application, even with extremely deep recursion. Using half the size
> >    maintains adequate while being more resource-efficient margin
> >
> > 2. On memory-constrained systems (e.g., platforms with only 4GB of
> >    physical memory, which is a common configuration), allocating 4GB
> >    of virtual address space for shadow stack per process/thread can
> >    lead to virtual memory allocation failures when the overcommit mode
> >    is set to OVERCOMMIT_GUESS or OVERCOMMIT_NEVER:
> >    Error: "__vm_enough_memory: not enough memory for the allocation"
> >
> > This reduces virtual address space consumption by 50% while maintaining
> > more than adequate space for return address storage.
> >
> > Signed-off-by: Zong Li <zong.li at sifive.com>
> > ---
> >  arch/riscv/kernel/usercfi.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> > index 6eaa0d94fdfe..4a48dd28d113 100644
> > --- a/arch/riscv/kernel/usercfi.c
> > +++ b/arch/riscv/kernel/usercfi.c
> > @@ -109,15 +109,16 @@ void set_indir_lp_lock(struct task_struct *task, bool lock)
> >       task->thread_info.user_cfi_state.ufcfi_locked = lock;
> >  }
> >  /*
> > - * If size is 0, then to be compatible with regular stack we want it to be as big as
> > - * regular stack. Else PAGE_ALIGN it and return back
> > + * The shadow stack only stores the return address and not any variables
> > + * this should be more than sufficient for most applications.
> > + * Else PAGE_ALIGN it and return back
> >   */
> >  static unsigned long calc_shstk_size(unsigned long size)
> >  {
> >       if (size)
> >               return PAGE_ALIGN(size);
> >
> > -     return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G));
> > +     return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK) / 2, SZ_2G));
>
> Use min() instead of min_t().
> All the values (before and after the patch) are unsigned.

Thanks. I will modify it in the next version.

>
>         David
>
>
> >  }
> >
> >  /*
>