linux-kernel - [PATCH 01/24] mm/swap: fix a potential undefined behavior issue

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Date:   Mon, 20 Nov 2023 03:47:17 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        Kairui Song <kasong@...cent.com>
Subject: [PATCH 01/24] mm/swap: fix a potential undefined behavior issue

From: Kairui Song <kasong@...cent.com>

When folio is NULL, taking the address of its struct member is an
undefined behavior, the UB is caused by applying -> operator
to a pointer not pointing to any object. Although in practice this
won't lead to a real issue, still better to fix it, also makes the
code less error-prone, when folio is NULL, page is also NULL,
instead of a meanless offset value.

Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index e27e2e5beb3f..70ffa867b1be 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3861,7 +3861,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 			/* skip swapcache */
 			folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
 						vma, vmf->address, false);
-			page = &folio->page;
 			if (folio) {
 				__folio_set_locked(folio);
 				__folio_set_swapbacked(folio);
@@ -3879,6 +3878,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 					workingset_refault(folio, shadow);
 
 				folio_add_lru(folio);
+				page = &folio->page;
 
 				/* To provide entry to swap_readpage() */
 				folio->swap = entry;
-- 
2.42.0