Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
From:   Kairui Song <ryncsn@gmail.com>
To:     linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Huang, Ying" <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org,
        Kairui Song <kasong@tencent.com>
Subject: [PATCH 01/24] mm/swap: fix a potential undefined behavior issue
Date:   Mon, 20 Nov 2023 03:47:17 +0800
Message-ID: <20231119194740.94101-2-ryncsn@gmail.com>
In-Reply-To: <20231119194740.94101-1-ryncsn@gmail.com>
References: <20231119194740.94101-1-ryncsn@gmail.com>
Reply-To: Kairui Song <kasong@tencent.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

From: Kairui Song <kasong@tencent.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@tencent.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