Re: [PATCH 1/8] mm/fs: don't allow writes to immutable files

[Theodore Ts'o]

On Wed, Apr 17, 2019 at 12:04:33PM -0700, Darrick J. Wong wrote:
> diff --git a/mm/memory.c b/mm/memory.c
> index ab650c21bccd..dfd5eba278d6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2149,6 +2149,9 @@ static vm_fault_t do_page_mkwrite(struct vm_fault *vmf)
>  
>       vmf->flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;
>  
> +     if (vmf->vma->vm_file && IS_IMMUTABLE(file_inode(vmf->vma->vm_file)))
> +             return VM_FAULT_SIGBUS;
> +
>       ret = vmf->vma->vm_ops->page_mkwrite(vmf);
>       /* Restore original flags so that caller is not surprised */
>       vmf->flags = old_flags;

Shouldn't this check be moved before the modification of vmf->flags?
It looks like do_page_mkwrite() isn't supposed to be returning with
vmf->flags modified, lest "the caller gets surprised".

                                           - Ted