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Commit 2d033eaa authored by Maxim Patlasov's avatar Maxim Patlasov Committed by Miklos Szeredi
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fuse: fix race in fuse_writepages() 



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.

Signed-off-by: default avatarMaxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: default avatarMiklos Szeredi <mszeredi@suse.cz>
parent 26d614df

fs/fuse/file.c

+16 −1
Original line number Diff line number Diff line
@@ -1590,6 +1590,7 @@ 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)
@@ -1598,12 +1599,17 @@ 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,
@@ -1684,7 +1690,7 @@ 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
@@ -1716,6 +1722,13 @@ 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 */
@@ -1725,6 +1738,8 @@ static int fuse_writepages(struct address_space *mapping, 
	}

	if (data.ff)

		fuse_file_put(data.ff, false);



	kfree(data.orig_pages);

out:

	return err;

}