Re: [PATCH RFC/RFT v2 1/2] mm: helper `is_shadow_stack_vma` to check shadow stack vma

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Wed, 2024-10-16 at 14:57 -0700, Deepak Gupta wrote:
> VM_SHADOW_STACK (alias to VM_HIGH_ARCH_5) is used to encode shadow stack
> VMA on three architectures (x86 shadow stack, arm GCS and RISC-V shadow
> stack). In case architecture doesn't implement shadow stack, it's VM_NONE
> Introducing a helper `is_shadow_stack_vma` to determine shadow stack vma
> or not.

I don't understand why we need this. IIRC was some discussion about wanting to
abstract different ways of testing for shadow stack VMAs for different
architectures since risc-v was going to do it differently. But now this says
they are all the same, so what's wrong with the open coded check?

> 
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> ---
>  mm/gup.c  |  2 +-
>  mm/mmap.c |  2 +-
>  mm/vma.h  | 10 +++++++---
>  3 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index a82890b46a36..8e6e14179f6c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1282,7 +1282,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>  		    !writable_file_mapping_allowed(vma, gup_flags))
>  			return -EFAULT;
>  
> -		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
> +		if (!(vm_flags & VM_WRITE) || is_shadow_stack_vma(vm_flags)) {
>  			if (!(gup_flags & FOLL_FORCE))
>  				return -EFAULT;
>  			/* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
> diff --git a/mm/mmap.c b/mm/mmap.c
> index dd4b35a25aeb..0853e6784069 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -708,7 +708,7 @@ static unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info)
>   */
>  static inline unsigned long stack_guard_placement(vm_flags_t vm_flags)
>  {
> -	if (vm_flags & VM_SHADOW_STACK)
> +	if (is_shadow_stack_vma(vm_flags))
>  		return PAGE_SIZE;
>  
>  	return 0;
> diff --git a/mm/vma.h b/mm/vma.h
> index 819f994cf727..0f238dc37231 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -357,7 +357,7 @@ static inline struct vm_area_struct *vma_prev_limit(struct vma_iterator *vmi,
>  }
>  
>  /*
> - * These three helpers classifies VMAs for virtual memory accounting.
> + * These four helpers classifies VMAs for virtual memory accounting.
>   */
>  
>  /*
> @@ -368,6 +368,11 @@ static inline bool is_exec_mapping(vm_flags_t flags)
>  	return (flags & (VM_EXEC | VM_WRITE | VM_STACK)) == VM_EXEC;
>  }
>  
> +static inline bool is_shadow_stack_vma(vm_flags_t vm_flags)
> +{
> +	return !!(vm_flags & VM_SHADOW_STACK);
> +}
> +
>  /*
>   * Stack area (including shadow stacks)
>   *
> @@ -376,7 +381,7 @@ static inline bool is_exec_mapping(vm_flags_t flags)
>   */
>  static inline bool is_stack_mapping(vm_flags_t flags)
>  {
> -	return ((flags & VM_STACK) == VM_STACK) || (flags & VM_SHADOW_STACK);
> +	return ((flags & VM_STACK) == VM_STACK) || is_shadow_stack_vma(flags);
>  }
>  
>  /*
> @@ -387,7 +392,6 @@ static inline bool is_data_mapping(vm_flags_t flags)
>  	return (flags & (VM_WRITE | VM_SHARED | VM_STACK)) == VM_WRITE;
>  }
>  
> -
>  static inline void vma_iter_config(struct vma_iterator *vmi,
>  		unsigned long index, unsigned long last)
>  {
>