[bug report] staging: erofs: tidy up decompression frontend

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Gao Xiang,

This is a semi-automatic email about new static checker warnings.

The patch 97e86a858bc3: "staging: erofs: tidy up decompression
frontend" from Jul 31, 2019, leads to the following Smatch complaint:

    fs/erofs/zdata.c:670 z_erofs_do_read_page()
    error: we previously assumed 'clt->cl' could be null (see line 596)

fs/erofs/zdata.c
   595			/* didn't get a valid collection previously (very rare) */
   596			if (!clt->cl)
                            ^^^^^^^^
New NULL check.

   597				goto restart_now;
   598			goto hitted;
   599		}
   600	
   601		/* go ahead the next map_blocks */
   602		debugln("%s: [out-of-range] pos %llu", __func__, offset + cur);
   603	
   604		if (z_erofs_collector_end(clt))
   605			fe->backmost = false;
   606	
   607		map->m_la = offset + cur;
   608		map->m_llen = 0;
   609		err = z_erofs_map_blocks_iter(inode, map, 0);
   610		if (unlikely(err))
   611			goto err_out;
   612	
   613	restart_now:
   614		if (unlikely(!(map->m_flags & EROFS_MAP_MAPPED)))
   615			goto hitted;
   616	
   617		err = z_erofs_collector_begin(clt, inode, map);
   618		if (unlikely(err))
   619			goto err_out;
   620	
   621		/* preload all compressed pages (maybe downgrade role if necessary) */
   622		if (should_alloc_managed_pages(fe, sbi->cache_strategy, map->m_la))
   623			cache_strategy = DELAYEDALLOC;
   624		else
   625			cache_strategy = DONTALLOC;
   626	
   627		preload_compressed_pages(clt, MNGD_MAPPING(sbi),
   628					 cache_strategy, pagepool);
   629	
   630		tight &= (clt->mode >= COLLECT_PRIMARY_HOOKED);
   631	hitted:
   632		cur = end - min_t(unsigned int, offset + end - map->m_la, end);
   633		if (unlikely(!(map->m_flags & EROFS_MAP_MAPPED))) {
   634			zero_user_segment(page, cur, end);
   635			goto next_part;
   636		}
   637	
   638		/* let's derive page type */
   639		page_type = cur ? Z_EROFS_VLE_PAGE_TYPE_HEAD :
   640			(!spiltted ? Z_EROFS_PAGE_TYPE_EXCLUSIVE :
   641				(tight ? Z_EROFS_PAGE_TYPE_EXCLUSIVE :
   642					Z_EROFS_VLE_PAGE_TYPE_TAIL_SHARED));
   643	
   644		if (cur)
   645			tight &= (clt->mode >= COLLECT_PRIMARY_FOLLOWED);
   646	
   647	retry:
   648		err = z_erofs_attach_page(clt, page, page_type);
   649		/* should allocate an additional staging page for pagevec */
   650		if (err == -EAGAIN) {
   651			struct page *const newpage =
   652				__stagingpage_alloc(pagepool, GFP_NOFS);
   653	
   654			err = z_erofs_attach_page(clt, newpage,
   655						  Z_EROFS_PAGE_TYPE_EXCLUSIVE);
   656			if (likely(!err))
   657				goto retry;
   658		}
   659	
   660		if (unlikely(err))
   661			goto err_out;
   662	
   663		index = page->index - (map->m_la >> PAGE_SHIFT);
   664	
   665		z_erofs_onlinepage_fixup(page, index, true);
   666	
   667		/* bump up the number of spiltted parts of a page */
   668		++spiltted;
   669		/* also update nr_pages */
   670		clt->cl->nr_pages = max_t(pgoff_t, clt->cl->nr_pages, index + 1);
                ^^^^^^^^^^^^^^^^^                  ^^^^^^^^^^^^^^^^^
Unchecked dereferences.

   671	next_part:
   672		/* can be used for verification */

regards,
dan carpenter