[PATCH v5] riscv: cif: reduce shadow stack size limit from 4GB to 512MB

[Zong Li]

On Tue, May 19, 2026 at 5:20 PM Zong Li <zong.li at sifive.com> wrote:
>
> On Tue, May 19, 2026 at 4:28 PM Ron Economos <re at w6rz.net> wrote:
> >
> > On 5/19/26 00:18, Zong Li wrote:
> > > Rationale:
> > >
> > > 1. Shadow stacks only store return addresses (8 bytes per entry), not
> > >     local variables, function parameters, or saved registers. A 512MB
> > >     shadow stack is far more than sufficient for any practical
> > >     application, even with extremely deep recursion. This 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"
> > >
> > > Suggested-by: David Laight <david.laight.linux at gmail.com>
> > > Signed-off-by: Zong Li <zong.li at sifive.com>
> > > ---
> > >
> > > Changed in v4:
> > > - Fix wrong subject. It is 512MB instead of 2GB
> > >
> > > Changed in v3:
> > > - Remove max(). PAGE_ALIGN() already rounds up
> > > - Change stack size to RLIMIT_STACK/8 with SZ_512M cap. Suggested by David Laight
> > >
> > > Changed in v2:
> > > - Add max() in case RLIMIT_STACK is smaller than PAGE_SIZE. Suggested by
> > >    Paul Walmsley and Sashiko
> > >
> > > Changed in v1:
> > > - Use min() instead of min_t(). Suggested by David Laight
> > >
> > >   arch/riscv/kernel/usercfi.c | 6 +++---
> > >   1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> > > index 6eaa0d94fdfe..2036918a77db 100644
> > > --- a/arch/riscv/kernel/usercfi.c
> > > +++ b/arch/riscv/kernel/usercfi.c
> > > @@ -109,15 +109,15 @@ 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
> > > + * 512M should be more than sufficient for most applications.
> > >    */
> > >   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(rlimit(RLIMIT_STACK) / 8, SZ_512M));
> > >   }
> > >
> > >   /*
> >
> > Just FYI, your V2 version of this patch was merged in Linux 7.1-rc4 (commit 6c7674b5b7ae513cecae22aa9dcdcf533862cf5c). You need to
> > rebase, otherwise this patch won't apply.
>

Hi David,
Since the original patch has been merged, would you like to send a new
patch from your side to adjust the size? Or do you prefer that I do
that for you? Please let me know your thoughts. Thanks

> Thank you for pointing this out. Perhaps let's drop this series and
> send a new one to explain why we want to reduce the size to 512MB
>
> >