linux-kernel - Re: [PATCH 1/3] vfs: dedpue: return s64

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Date:   Mon, 7 May 2018 05:17:58 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [PATCH 1/3] vfs: dedpue: return s64

On Mon, May 07, 2018 at 01:32:09PM +0200, Miklos Szeredi wrote:
> On Mon, May 7, 2018 at 1:11 PM, Matthew Wilcox <willy@...radead.org> wrote:
> > On Mon, May 07, 2018 at 10:21:06AM +0200, Miklos Szeredi wrote:
> >> f_op->dedupe_file_range() gets a u64 length to dedup and returns an ssize_t
> >> actual length deduped.  This breaks badly on 32bit archs since the returned
> >> length will be truncated and possibly overflow into the sign bit (xfs and
> >> ocfs2 are affected, btrfs limits actual length to 16MiB).
> >>
> >> Returning s64 should be good, since clone_verify_area() makes sure that the
> >> supplied length doesn't overflow.
> >
> > Why s64 rather than loff_t?  Particularly since the next patch turns
> > the paramters into loff_t.
> 
> Next patch turns the offsets into loff_t and leaves "len" as u64.  A
> size is definitely not an offset, I'd consider changing the type of
> "len" to loff_t a misuse.

Usually a size is the size of something in memory.  The length of
something on storage is definitely an loff_t.  Look at fallocate()
for an example.  You could also argue that lseek where whence is set to
anything other than SEEK_SET is also being used as a length rather than
an absolute offset.  We also already use 'loff_t len' as an argument to
vfs_dedupe_file_range_compare().