linux-kernel - Re: [PATCH 01/16] mm: use untagged_addr() for get_user_pages

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Message-ID: <20190621133911.GL19891@ziepe.ca>
Date:   Fri, 21 Jun 2019 10:39:11 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Nicholas Piggin <npiggin@...il.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-mips@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/16] mm: use untagged_addr() for get_user_pages_fast
 addresses

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 <hch@....de>
>  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.