Re: [PATCH v12 06/28] riscv/mm : ensure PROT_WRITE leads to VM_READ | VM_WRITE

[Radim Krčmář]

2025-03-14T14:39:25-07:00, Deepak Gupta <debug@xxxxxxxxxxxx>:
> diff --git a/arch/riscv/include/asm/mman.h b/arch/riscv/include/asm/mman.h
> +static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
> +						   unsigned long pkey __always_unused)
> +{
> +	unsigned long ret = 0;
> +
> +	/*
> +	 * If PROT_WRITE was specified, force it to VM_READ | VM_WRITE.
> +	 * Only VM_WRITE means shadow stack.
> +	 */

This function also changes PROT_WX to VM_RWX, which is effectively not
changing anything, but I think it deserves an explicit intent.
(At least in the commit message.)

> +	if (prot & PROT_WRITE)
> +		ret = (VM_READ | VM_WRITE);
> +	return ret;
> +}
> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> @@ -16,6 +17,15 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
> +	/*
> +	 * If PROT_WRITE is specified then extend that to PROT_READ
> +	 * protection_map[VM_WRITE] is now going to select shadow stack encodings.
> +	 * So specifying PROT_WRITE actually should select protection_map [VM_WRITE | VM_READ]
> +	 * If user wants to create shadow stack then they should use `map_shadow_stack` syscall.
> +	 */
> +	if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
> +		prot |= PROT_READ;

Why isn't the previous hunk be enough?  (Or why don't we do just this?)

riscv_sys_mmap() eventually calls arch_calc_vm_prot_bits(), so I'd
rather fix each code path just once.

Thanks.