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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 3 Mar 2020 15:10:50 +0100
Message-ID: <CAJfpegtFyZqSRzo3uuXp1S2_jJJ29DL=xAwKjpEGvyG7=AzabA@mail.gmail.com>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Karel Zak <kzak@redhat.com>, David Howells <dhowells@redhat.com>,
        Ian Kent <raven@themaw.net>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
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On Tue, Mar 3, 2020 at 2:43 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Tue, Mar 03, 2020 at 02:34:42PM +0100, Miklos Szeredi wrote:

> > If buffer is too small to fit the whole file, return error.
>
> Why?  What's wrong with just returning the bytes asked for?  If someone
> only wants 5 bytes from the front of a file, it should be fine to give
> that to them, right?

I think we need to signal in some way to the caller that the result
was truncated (see readlink(2), getxattr(2), getcwd(2)), otherwise the
caller might be surprised.

>
> > Verify that the number of bytes read matches the file size, otherwise
> > return error (may need to loop?).
>
> No, we can't "match file size" as sysfs files do not really have a sane
> "size".  So I don't want to loop at all here, one-shot, that's all you
> get :)

Hmm.  I understand the no-size thing.  But looping until EOF (i.e.
until read return zero) might be a good idea regardless, because short
reads are allowed.

Thanks,
Miklos