Subject: Re: [PATCH 01/16] mm: use untagged_addr() for get_user_pages_fast addresses

From: Jason Gunthorpe <[email-masked]>
Date: Fri, 21 Jun 2019 10:39:11 -0300

On Tue, Jun 11, 2019 at 04:40:47PM +0200, Christoph Hellwig wrote:
> This will allow sparc64 to override its ADI tags for
> get_user_pages and get_user_pages_fast.
> 
> Signed-off-by: Christoph Hellwig <[email-masked]>
>  mm/gup.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index ddde097cf9e4..6bb521db67ec 100644
> +++ b/mm/gup.c
> @@ -2146,7 +2146,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	unsigned long flags;
>  	int nr = 0;
>  
> -	start &= PAGE_MASK;
> +	start = untagged_addr(start) & PAGE_MASK;
>  	len = (unsigned long) nr_pages << PAGE_SHIFT;
>  	end = start + len;

Hmm, this function, and the other, goes on to do:

        if (unlikely(!access_ok((void __user *)start, len)))
                return 0;

and I thought that access_ok takes in the tagged pointer?

How about re-order it a bit?

diff --git a/mm/gup.c b/mm/gup.c
index ddde097cf9e410..f48747ced4723b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2148,11 +2148,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 
 	start &= PAGE_MASK;
 	len = (unsigned long) nr_pages << PAGE_SHIFT;
-	end = start + len;
-
 	if (unlikely(!access_ok((void __user *)start, len)))
 		return 0;
 
+	start = untagged_ptr(start);
+	end = start + len;
+
 	/*
 	 * Disable interrupts.  We use the nested form as we can already have
 	 * interrupts disabled by get_futex_key.