linux-kernel - Re: [PATCH v4] fs: introduce is_dot_or

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Date:   Tue, 10 Dec 2019 23:10:09 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        linux-fsdevel@...r.kernel.org, ecryptfs@...r.kernel.org,
        linux-fscrypt@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] fs: introduce is_dot_or_dotdot helper for cleanup

On Tue, Dec 10, 2019 at 11:19:13AM -0800, Eric Biggers wrote:

> > +static inline bool is_dot_or_dotdot(const unsigned char *name, size_t len)
> > +{
> > +	if (unlikely(name[0] == '.')) {
> > +		if (len < 2 || (len == 2 && name[1] == '.'))
> > +			return true;
> > +	}
> > +
> > +	return false;
> > +}
> 
> This doesn't handle the len=0 case.  Did you check that none of the users pass
> in zero-length names?  It looks like fscrypt_fname_disk_to_usr() can, if the
> directory entry on-disk has a zero-length name.  Currently it will return
> -EUCLEAN in that case, but with this patch it may think it's the name ".".
> 
> So I think there needs to either be a len >= 1 check added, *or* you need to
> make an argument for why it's okay to not care about the empty name case.

Frankly, the only caller that matters in practice is link_path_walk(); _that_
is by far the hottest path that might make use of that thing.

BTW, the callers that might end up passing 0 for len really ought to take
a good look at another thing - that name[0] is, in fact, mapped.  Something
along the lines of
	if (name + len > end_of_buffer)
		sod off
	if (<that function>(name, len))
		....
is not enough, for obvious reasons.