linux-kernel - Re: [PATCH] selftests: secretmem: Floor the memory size to the multiple of page

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Message-ID: <20231215105933.GO636165@kernel.org>
Date: Fri, 15 Dec 2023 12:59:33 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Shuah Khan <shuah@...nel.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	kernel@...labora.com, "kernelci.org bot" <bot@...nelci.org>,
	linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests: secretmem: Floor the memory size to the
 multiple of page_size

On Thu, Dec 14, 2023 at 03:19:30PM +0500, Muhammad Usama Anjum wrote:
> The "locked-in-memory size" limit per process can be non-multiple of
> page_size. The mmap() fails if we try to allocate locked-in-memory
> with same size as the allowed limit if it isn't multiple of the
> page_size because mmap() rounds off the memory size to be allocated
> to next multiple of page_size.
> 
> Fix this by flooring the length to be allocated with mmap() to the
> previous multiple of the page_size.
> 
> Fixes: 76fe17ef588a ("secretmem: test: add basic selftest for memfd_secret(2)")
> Reported-by: "kernelci.org bot" <bot@...nelci.org>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> ---
>  tools/testing/selftests/mm/memfd_secret.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
> index 957b9e18c729..9b298f6a04b3 100644
> --- a/tools/testing/selftests/mm/memfd_secret.c
> +++ b/tools/testing/selftests/mm/memfd_secret.c
> @@ -62,6 +62,9 @@ static void test_mlock_limit(int fd)
>  	char *mem;
>  
>  	len = mlock_limit_cur;
> +	if (len % page_size != 0)
> +		len = (len/page_size) * page_size;
> +

With mlock limit smaller than a page we get zero length here and mmap will
fail with -EINVAL because of it.
In this case I think we can just skip the first mmap and only check that
mmaping more than mlock limit fails.

>  	mem = mmap(NULL, len, prot, mode, fd, 0);
>  	if (mem == MAP_FAILED) {
>  		fail("unable to mmap secret memory\n");
> -- 
> 2.42.0
> 

-- 
Sincerely yours,
Mike.