linux-kernel - Re: [patch] hfs: fix hfs

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Date:   Mon, 16 Jan 2017 17:22:11 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Viacheslav Dubeyko <slava@...eyko.com>
Cc:     Chengyu Song <csong84@...ech.edu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [patch] hfs: fix hfs_readdir()

I was reviewing old warnings and I stumbled across this one again.
Although I wrote that &fd.key->cat and "fd.key" are equivalent, I feel
that actually we should be doing the former.  fd.key is a union but we
want the ->cat member of the union.

On Tue, Jan 26, 2016 at 01:54:06PM -0800, Viacheslav Dubeyko wrote:
> On Tue, 2016-01-26 at 22:18 +0300, Dan Carpenter wrote:
> > Hm, I completely didn't see that it was a union instead of a struct.  I
> > still think my fix is actually correct though.  Now that you point out
> > the union, I see that my change is equivalent to just removing the '&'
> > char.
> > 
> > -	memcpy(&rd->key, &fd.key, sizeof(struct hfs_cat_key));
> > +	memcpy(&rd->key, fd.key, sizeof(struct hfs_cat_key));
> > 
> 
> Yeahh, it looks correct right now. The rd is the pointer that includes
> struct hfs_cat_key object. So, we need to use &rd->key. But on another
> side we have struct hfs_find_data object on the stack. And this object
> includes the pointer on union btree_key. We want to copy struct
> hfs_cat_key object and we should use sizeof(struct hfs_cat_key).

I've read this paragraph several times now and I think you are saying
that the patch is correct.

> 
> > We don't want to copy sizeof(*fd.key) because that would write past the
> > end of the destination struct.
> > 
> > On Tue, Jan 26, 2016 at 10:18:56AM -0800, Viacheslav Dubeyko wrote:
> > > Another worry could be the "search_key" field of the struct
> > > hfs_find_data.
> > 
> > I don't understand what you mean here.
> > 
> 
> I mean here that we could have another incorrect copy operations for
> "search_key" field. That's all.

I don't see the bugs you are saying might exist...  ;)

regards,
dan carpenter