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

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Message-ID: <CAOssrKds6zYpoN88ns2W3c+uqSJ_iWFRjsuMM-iAv3MqU15GDA@mail.gmail.com>
Date:   Mon, 7 May 2018 16:26:05 +0200
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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 7, 2018 at 2:17 PM, Matthew Wilcox <willy@...radead.org> wrote:
> 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().

Fair enough.  Will fix.

Thanks,
Miklos