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

[David Laight <david.laight.linux@gmail.com>]

On Thu, 14 May 2026 00:50:35 -0700
Zong Li <zong.li@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.
> 
> Additionally, add max(PAGE_SIZE, size) constraint, which covers the
> case where RLIMIT_STACK is smaller than PAGE_SIZE.
> 
> Signed-off-by: Zong Li <zong.li@sifive.com>
> ---
> 
> 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 | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> index 6eaa0d94fdfe..0f75e8f5d0ec 100644
> --- a/arch/riscv/kernel/usercfi.c
> +++ b/arch/riscv/kernel/usercfi.c
> @@ -109,15 +109,17 @@ 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
> + * 2G 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));
> +	size = PAGE_ALIGN(min(rlimit(RLIMIT_STACK) / 2, SZ_2G));

PAGE_ALIGN() already rounds up, so the only problem would be if rlimit(STACK)
were 0 or 1, I'm sure that would fail earlier (or not be allowed).

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.

-- David

> +
> +	return max(PAGE_SIZE, size);
>  }
>  
>  /*