linux-kernel - Re: [PATCH v2 6/5] statx: add STATX_RESULT

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Message-Id: <191D94B6-B91B-45C0-81A2-C6E7A3124731@dilger.ca>
Date:   Fri, 19 Oct 2018 23:55:04 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     David Howells <dhowells@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Florian Weimer <fw@...eb.enyo.de>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [PATCH v2 6/5] statx: add STATX_RESULT_MASK flag

On Oct 19, 2018, at 11:42 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
>>> +#define STATX_RESULT_MASK STATX__RESERVED
>> 
>> Please don't use that bit.
> 
> Using it internally is perfectly harmless.   If we'll need to extend
> statx in the future and make use of this flag externally, then we can
> easily move the internal flag somewhere else (e.g. extend request_mask
> to 64bit, which we'll probably need to do anyway in that case).

I was thinking about this - what is the point of returning an error
if STATX__RESERVED is set?  If this is used to indicate the presence
of e.g. stx_mask2, then newer applications trying to request any of the
flags encoded into stx_mask2 will get an error, rather than the expected
behaviour of "ignore flags you don't understand, and don't set them in
the return stx_mask".

Essentially, this will make STATX__RESERVED useless in the future, since
no application will be able to use it without getting an error if they
are running on an old kernel.

Cheers, Andreas

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