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From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Baoquan He <bhe@redhat.com>, Christoph Hellwig <hch@infradead.org>,
        Uladzislau Rezki <urezki@gmail.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>
Subject: [PATCH] lib/test_vmalloc.c: avoid garbage in page array
Date:   Wed, 24 May 2023 09:24:24 +0100
Message-Id: <20230524082424.10022-1-lstoakes@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

It turns out that alloc_pages_bulk_array() does not treat the page_array
parameter as an output parameter, but rather reads the array and skips any
entries that have already been allocated.

This is somewhat unexpected and breaks this test, as we allocate the pages
array uninitialised on the assumption it will be overwritten.

As a result, the test was referencing uninitialised data and causing the
PFN to not be valid and thus a WARN_ON() followed by a null pointer deref
and panic.

In addition, this is an array of pointers not of struct page objects, so we
need only allocate an array with elements of pointer size.

We solve both problems by simply using kcalloc() and referencing
sizeof(struct page *) rather than sizeof(struct page).

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 lib/test_vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c
index 9dd9745d365f..3718d9886407 100644
--- a/lib/test_vmalloc.c
+++ b/lib/test_vmalloc.c
@@ -369,7 +369,7 @@ vm_map_ram_test(void)
 	int i;
 
 	map_nr_pages = nr_pages > 0 ? nr_pages:1;
-	pages = kmalloc(map_nr_pages * sizeof(struct page), GFP_KERNEL);
+	pages = kcalloc(map_nr_pages, sizeof(struct page *), GFP_KERNEL);
 	if (!pages)
 		return -1;
 
-- 
2.40.1