linux-kernel - Re: make getdents/readdir POSIX compliant wrt mount-point dirent.d

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Date:	Wed, 4 Nov 2009 15:22:34 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	Eric Blake <ebb9@....net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: make getdents/readdir POSIX compliant wrt mount-point 
 dirent.d_ino

On Thu, 3 Sep 2009 14:50:52 +0000 (UTC)
Eric Blake <ebb9@....net> wrote:

> Ulrich Drepper <drepper <at> gmail.com> writes:
> 
> > I guess that this is really a difficult way to solve.  I wouldn't want
> > to pay for something which is hardly ever really used.
> > 
> > But there are programs out there which would like to use the inode
> > uniqueness.  Therefore the next best thing to do is perhaps to return
> > a flag in the getdents information (in d_type, perhaps) to indicate
> > that this is a mount point and/or that there are multiple ways to
> > access the file in question.  Then programs which can use the inode
> > information can be watching for this flag and enter the slow path only
> > if it's set.
> 
> An alternative to a flag in d_type might be setting d_ino to a sentinel value
> (there is plenty of existing code that refuses to use a readdir entry with d_ino
> of 0, so it would have to be something else; but maybe -1 would work).  But I
> definitely like your idea of making it obvious to the application (whether by
> d_ino or d_type) when it is necessary to use lstat to validate the inode number,
> and promising that if the flag is not set then d_ino is correct.
> 

The only problem with those schemes is that they assume that you're
using getdents. If you just lstat something then it's not clear.

It seems like a better scheme might be to do something that makes this
clear when using an lstat call too. Maybe we could coopt a bit in the
st_mode?

S_IFXDEV          020000

That's outside of the S_IFMT mask, so it shouldn't break S_IS* checks,
right?

-- 
Jeff Layton <jlayton@...hat.com>
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