Re: [PATCH] fuse: fix race in fuse_writepages()

[Miklos Szeredi]

On Fri, Aug 16, 2013 at 03:51:41PM +0400, Maxim Patlasov wrote:
> The patch is for
> 
>  git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git writepages.v2
> 
> The patch fixes a race between ftruncate(2), mmap-ed write and write(2):
> 
> 1) An user makes a page dirty via mmap-ed write.
> 2) The user performs shrinking truncate(2) intended to purge the page.
> 3) Before fuse_do_setattr calls truncate_pagecache, the page goes to
>    writeback. fuse_writepages_fill attaches a new page to FUSE_WRITE request,
>    then releases the original page by end_page_writeback and unlock it.
> 4) fuse_do_setattr completes and successfully returns. Since now, i_mutex
>    is free.
> 5) Ordinary write(2) extends i_size back to cover the page. Note that
>    fuse_send_write_pages do wait for fuse writeback, but for another
>    page->index.
> 6) fuse_writepages_fill attaches more pages to the request (if any), then
>    fuse_writepages_send is eventually called. It is supposed to crop
>    inarg->size of the request, but it doesn't because i_size has already been
>    extended back.
> 
> Moving end_page_writeback behind fuse_writepages_send guarantees that
> __fuse_release_nowrite (called from fuse_do_setattr) will crop inarg->size
> of the request before write(2) gets the chance to extend i_size.

Thanks for the report.  Your analysis looks correct.

Just one nit, why orig_pages? req->pages is already there, so why duplicate it?

Note: you can do __fuse_get_request()/fuse_put_request() to prevent the req from
going away after it's been sent.

Thanks,
Miklos


> 
> Signed-off-by: Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx>
> ---
>  fs/fuse/file.c |   17 ++++++++++++++++-
>  1 files changed, 16 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 568e859..0ebcc79 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1583,6 +1583,7 @@ struct fuse_fill_wb_data {
>  	struct fuse_req *req;
>  	struct fuse_file *ff;
>  	struct inode *inode;
> +	struct page **orig_pages;
>  };
>  
>  static void fuse_writepages_send(struct fuse_fill_wb_data *data)
> @@ -1591,12 +1592,17 @@ static void fuse_writepages_send(struct fuse_fill_wb_data *data)
>  	struct inode *inode = data->inode;
>  	struct fuse_conn *fc = get_fuse_conn(inode);
>  	struct fuse_inode *fi = get_fuse_inode(inode);
> +	int num_pages = req->num_pages;
> +	int i;
>  
>  	req->ff = fuse_file_get(data->ff);
>  	spin_lock(&fc->lock);
>  	list_add_tail(&req->list, &fi->queued_writes);
>  	fuse_flush_writepages(inode);
>  	spin_unlock(&fc->lock);
> +
> +	for (i = 0; i < num_pages; i++)
> +		end_page_writeback(data->orig_pages[i]);
>  }
>  
>  static int fuse_writepages_fill(struct page *page,
> @@ -1677,7 +1683,7 @@ static int fuse_writepages_fill(struct page *page,
>  
>  	inc_bdi_stat(page->mapping->backing_dev_info, BDI_WRITEBACK);
>  	inc_zone_page_state(tmp_page, NR_WRITEBACK_TEMP);
> -	end_page_writeback(page);
> +	data->orig_pages[req->num_pages] = page;
>  
>  	/*
>  	 * Protected by fc->lock against concurrent access by
> @@ -1709,6 +1715,13 @@ static int fuse_writepages(struct address_space *mapping,
>  	data.req = NULL;
>  	data.ff = NULL;
>  
> +	err = -ENOMEM;
> +	data.orig_pages = kzalloc(sizeof(struct page *) *
> +				  FUSE_MAX_PAGES_PER_REQ,
> +				  GFP_NOFS);
> +	if (!data.orig_pages)
> +		goto out;
> +
>  	err = write_cache_pages(mapping, wbc, fuse_writepages_fill, &data);
>  	if (data.req) {
>  		/* Ignore errors if we can write at least one page */
> @@ -1718,6 +1731,8 @@ static int fuse_writepages(struct address_space *mapping,
>  	}
>  	if (data.ff)
>  		fuse_file_put(data.ff, false);
> +
> +	kfree(data.orig_pages);
>  out:
>  	return err;
>  }
> 
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