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References: <20230524082424.10022-1-lstoakes@gmail.com>
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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Thu, 25 May 2023 22:04:11 +0200
Message-ID: <CA+KHdyVSD=vHh-WQxS_PrJ8GnBQZGFPgcS5AkmkyoUxpe_ziEw@mail.gmail.com>
Subject: Re: [PATCH] lib/test_vmalloc.c: avoid garbage in page array
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Baoquan He <bhe@redhat.com>,
        Christoph Hellwig <hch@infradead.org>
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On Wed, May 24, 2023 at 10:24=E2=80=AFAM Lorenzo Stoakes <lstoakes@gmail.co=
m> wrote:
>
> 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 an=
y
> entries that have already been allocated.
>
> This is somewhat unexpected and breaks this test, as we allocate the page=
s
> 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 =3D nr_pages > 0 ? nr_pages:1;
> -       pages =3D kmalloc(map_nr_pages * sizeof(struct page), GFP_KERNEL)=
;
> +       pages =3D kcalloc(map_nr_pages, sizeof(struct page *), GFP_KERNEL=
);
>         if (!pages)
>                 return -1;
>
> --
> 2.40.1
>
Uh.. :)

Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

--
Uladzislau Rezki