Re: [PATCH] ovl: fix lookup with middle layer opaque dir and absolute path redirects

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Mar 08, 2018 at 11:31:40PM +0200, Amir Goldstein wrote:
> The only way to explain the problem is with an example.
> Watch me make foo disappear:

Hi Amir,

I will look at it tomorrow more closely. I am also about to send 3 RFC
fixes for the issues we discussed. Please have a look and see what do you
think.

Vivek
> 
>  $ mkdir lower middle upper work work2 merged
>  $ mkdir lower/origin
>  $ touch lower/origin/foo
>  $ mount -t overlay none merged/ \
>          -olowerdir=lower,upperdir=middle,workdir=work2
>  $ mkdir merged/pure
>  $ mv merged/origin merged/pure/redirect
>  $ umount merged
>  $ mount -t overlay none merged/ \
>          -olowerdir=middle:lower,upperdir=upper,workdir=work
>  $ mv merged/pure/redirect merged/redirect
> 
> Now you see foo inside a twice redirected merged dir:
> 
>  $ ls merged/redirect
>  foo
>  $ umount merged
>  $ mount -t overlay none merged/ \
>          -olowerdir=middle:lower,upperdir=upper,workdir=work
> 
> After mount cycle you don't see foo inside the same dir:
> 
>  $ ls merged/redirect
> 
> During middle layer lookup, the opaqueness of middle/pure is left in
> the lookup state and then middle/pure/redirect is wrongly treated as
> opaque.
> 
> Fixes: 02b69b284cd7 ("ovl: lookup redirects")
> Cc: <stable@vger.kernel.org> #v4.10
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
> 
> Vivek,
> 
> This is the bug I was talking about in review of metacopy lookup.
> 
> Miklos,
> 
> This change seems a bit subtle... besides the reproducer I sanity tested
> redirects with unionmount-testsuite ./run --ov=10 and the overlay/quick
> xfstests. Neither would have hit this sort of corner case though.
> I will add a test to cover this case later on.
> 
> Thanks,
> Amir.
> 
>  fs/overlayfs/namei.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index 70fcfcc684cc..31ca563b7666 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -51,6 +51,14 @@ static int ovl_check_redirect(struct dentry *dentry, struct ovl_lookup_data *d,
>  	if (res == 0)
>  		goto invalid;
>  	if (buf[0] == '/') {
> +		/*
> +		 * One of the ancestor path elements in an absolute path
> +		 * lookup in ovl_lookup_layer() could have been opaque, but it
> +		 * possible for a decendant path element to reset opaqueness in
> +		 * case this path element has an absolute redirect to a lower
> +		 * layer.
> +		 */
> +		d->stop = d->opaque = false;
>  		for (s = buf; *s++ == '/'; s = next) {
>  			next = strchrnul(s, '/');
>  			if (s == next)
> -- 
> 2.7.4
>