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Sender: Guenter Roeck <groeck7@gmail.com>
Date:   Sat, 11 Jun 2022 06:43:57 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     David Howells <dhowells@redhat.com>
Cc:     jlayton@kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Mike Marshall <hubcap@omnibond.com>,
        Gao Xiang <xiang@kernel.org>, linux-afs@lists.infradead.org,
        v9fs-developer@lists.sourceforge.net, devel@lists.orangefs.org,
        linux-erofs@lists.ozlabs.org, linux-cachefs@redhat.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iov_iter: Fix iter_xarray_get_pages{,_alloc}()
Message-ID: <20220611134357.GA278954@roeck-us.net>
References: <165476202136.3999992.433442175457370240.stgit@warthog.procyon.org.uk>
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On Thu, Jun 09, 2022 at 09:07:01AM +0100, David Howells wrote:
> The maths at the end of iter_xarray_get_pages() to calculate the actual
> size doesn't work under some circumstances, such as when it's been asked to
> extract a partial single page.  Various terms of the equation cancel out
> and you end up with actual == offset.  The same issue exists in
> iter_xarray_get_pages_alloc().
> 
> Fix these to just use min() to select the lesser amount from between the
> amount of page content transcribed into the buffer, minus the offset, and
> the size limit specified.
> 
> This doesn't appear to have caused a problem yet upstream because network
> filesystems aren't getting the pages from an xarray iterator, but rather
> passing it directly to the socket, which just iterates over it.  Cachefiles
> *does* do DIO from one to/from ext4/xfs/btrfs/etc. but it always asks for
> whole pages to be written or read.
> 
> Fixes: 7ff5062079ef ("iov_iter: Add ITER_XARRAY")
> Reported-by: Jeff Layton <jlayton@kernel.org>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Alexander Viro <viro@zeniv.linux.org.uk>
> cc: Dominique Martinet <asmadeus@codewreck.org>
> cc: Mike Marshall <hubcap@omnibond.com>
> cc: Gao Xiang <xiang@kernel.org>
> cc: linux-afs@lists.infradead.org
> cc: v9fs-developer@lists.sourceforge.net
> cc: devel@lists.orangefs.org
> cc: linux-erofs@lists.ozlabs.org
> cc: linux-cachefs@redhat.com
> cc: linux-fsdevel@vger.kernel.org
> ---
> 
>  lib/iov_iter.c |   20 ++++----------------
>  1 file changed, 4 insertions(+), 16 deletions(-)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 834e1e268eb6..814f65fd0c42 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1434,7 +1434,7 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
>  {
>  	unsigned nr, offset;
>  	pgoff_t index, count;
> -	size_t size = maxsize, actual;
> +	size_t size = maxsize;
>  	loff_t pos;
>  
>  	if (!size || !maxpages)
> @@ -1461,13 +1461,7 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
>  	if (nr == 0)
>  		return 0;
>  
> -	actual = PAGE_SIZE * nr;
> -	actual -= offset;
> -	if (nr == count && size > 0) {
> -		unsigned last_offset = (nr > 1) ? 0 : offset;
> -		actual -= PAGE_SIZE - (last_offset + size);
> -	}
> -	return actual;
> +	return min(nr * PAGE_SIZE - offset, maxsize);

This needs min_t to avoid a build error on 32-bit builds.

In file included from include/linux/kernel.h:26,
                 from include/linux/crypto.h:16,
                 from include/crypto/hash.h:11,
                 from lib/iov_iter.c:2:
lib/iov_iter.c: In function 'iter_xarray_get_pages':
include/linux/minmax.h:20:35: error: comparison of distinct pointer types lacks a cast [-Werror]
...
lib/iov_iter.c:1628:16: note: in expansion of macro 'min'
 1628 |         return min(nr * PAGE_SIZE - offset, maxsize);
      |                ^~~

Guenter