Re: [PATCH 5/4] 9p: pass the correct prototype to read_cache_page

From: William Kucharski <william.kucharski@oracle.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Sami Tolvanen <samitolvanen@google.com>,
	Kees Cook <keescook@chromium.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-mtd@lists.infradead.org, linux-nfs@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 5/4] 9p: pass the correct prototype to read_cache_page
Date: Thu, 2 May 2019 00:08:29 -0600	[thread overview]
Message-ID: <AEBFD2FC-F94A-4E5B-8E1C-76380DDEB46E@oracle.com> (raw)
In-Reply-To: <20190501173443.GA19969@lst.de>

1) You need to pass "filp" rather than "filp->private_data" to read_cache_pages()
in v9fs_fid_readpage().

The patched code passes "filp->private_data" as the "data" parameter to
read_cache_pages(), which would generate a call to:

    filler(data, page)

which would become a call to:

static int v9fs_vfs_readpage(struct file *filp, struct page *page)
{	
        return v9fs_fid_readpage(filp->private_data, page);
}

which would then effectively become:

    v9fs_fid_readpage(filp->private_data->private_data, page)

Which isn't correct; because data is a void *, no error is thrown when
v9fs_vfs_readpages treats filp->private_data as if it is filp.

2) I'd also like to see an explicit comment in do_read_cache_page() along
the lines of:

/*
 * If a custom page filler was passed in use it, otherwise use the
 * standard readpage() routine defined for the address_space.
 *
 */

3) Patch 5/4?

Otherwise it looks good.

Reviewed-by: William Kucharski <william.kucharski@oracle.com>

> On May 1, 2019, at 11:34 AM, Christoph Hellwig <hch@lst.de> wrote:
> 
> Fix the callback 9p passes to read_cache_page to actually have the
> proper type expected.  Casting around function pointers can easily
> hide typing bugs, and defeats control flow protection.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/9p/vfs_addr.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
> index 0bcbcc20f769..02e0fc51401e 100644
> --- a/fs/9p/vfs_addr.c
> +++ b/fs/9p/vfs_addr.c
> @@ -50,8 +50,9 @@
>  * @page: structure to page
>  *
>  */
> -static int v9fs_fid_readpage(struct p9_fid *fid, struct page *page)
> +static int v9fs_fid_readpage(void *data, struct page *page)
> {
> +	struct p9_fid *fid = data;
> 	struct inode *inode = page->mapping->host;
> 	struct bio_vec bvec = {.bv_page = page, .bv_len = PAGE_SIZE};
> 	struct iov_iter to;
> @@ -122,7 +123,8 @@ static int v9fs_vfs_readpages(struct file *filp, struct address_space *mapping,
> 	if (ret == 0)
> 		return ret;
> 
> -	ret = read_cache_pages(mapping, pages, (void *)v9fs_vfs_readpage, filp);
> +	ret = read_cache_pages(mapping, pages, v9fs_fid_readpage,
> +			filp->private_data);
> 	p9_debug(P9_DEBUG_VFS, "  = %d\n", ret);
> 	return ret;
> }