linux-kernel - Re: [PATCH v1 2/3] selftests/memfd

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Date: Tue, 26 Mar 2024 08:17:59 +0200
From: Mike Rapoport <rppt@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <mszeredi@...hat.com>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	xingwei lee <xrivendell7@...il.com>,
	yue sun <samsun1006219@...il.com>
Subject: Re: [PATCH v1 2/3] selftests/memfd_secret: add vmsplice() test

Hi David,

On Mon, Mar 25, 2024 at 02:41:13PM +0100, David Hildenbrand wrote:
> Let's add a simple reproducer for a scneario where GUP-fast could succeed
> on secretmem folios, making vmsplice() succeed instead of failing. The
> reproducer is based on a reproducer [1] by Miklos Szeredi.
> 
> Perform the ftruncate() only once, and check the return value.
> 
> For some reason, vmsplice() reliably fails (making the test succeed) when
> we move the test_vmsplice() call after test_process_vm_read() /
> test_ptrace().

That's because ftruncate() call was in test_remote_access() and you need it
to mmap secretmem.

> Properly cleaning up in test_remote_access(), which is not
> part of this change, won't change that behavior. Therefore, run the
> vmsplice() test for now first -- something is a bit off once we involve
> fork().
> 
> [1] https://lkml.kernel.org/r/CAJfpegt3UCsMmxd0taOY11Uaw5U=eS1fE5dn0wZX3HF0oy8-oQ@mail.gmail.com
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  tools/testing/selftests/mm/memfd_secret.c | 44 +++++++++++++++++++++--
>  1 file changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
> index 9b298f6a04b3..0acbdcf8230e 100644
> --- a/tools/testing/selftests/mm/memfd_secret.c
> +++ b/tools/testing/selftests/mm/memfd_secret.c
> @@ -20,6 +20,7 @@
>  #include <unistd.h>
>  #include <errno.h>
>  #include <stdio.h>
> +#include <fcntl.h>
>  
>  #include "../kselftest.h"
>  
> @@ -83,6 +84,43 @@ static void test_mlock_limit(int fd)
>  	pass("mlock limit is respected\n");
>  }
>  
> +static void test_vmsplice(int fd)
> +{
> +	ssize_t transferred;
> +	struct iovec iov;
> +	int pipefd[2];
> +	char *mem;
> +
> +	if (pipe(pipefd)) {
> +		fail("pipe failed: %s\n", strerror(errno));
> +		return;
> +	}
> +
> +	mem = mmap(NULL, page_size, prot, mode, fd, 0);
> +	if (mem == MAP_FAILED) {
> +		fail("Unable to mmap secret memory\n");
> +		goto close_pipe;
> +	}
> +
> +	/*
> +	 * vmsplice() may use GUP-fast, which must also fail. Prefault the
> +	 * page table, so GUP-fast could find it.
> +	 */
> +	memset(mem, PATTERN, page_size);
> +
> +	iov.iov_base = mem;
> +	iov.iov_len = page_size;
> +	transferred = vmsplice(pipefd[1], &iov, 1, 0);
> +
> +	ksft_test_result(transferred < 0 && errno == EFAULT,
> +			 "vmsplice is blocked as expected\n");

The same message will be printed on success and on failure.

I think 

	if (transferred < 0 && errno == EFAULT)
		pass("vmsplice is blocked as expected");
	else
		fail("vmsplice: unexpected memory acccess");

is clearer than feeding different strings to ksft_test_result().

Other than that

Reviewed-by: Mike Rapoport (IBM) <rppt@...nel.org>

> +
> +	munmap(mem, page_size);
> +close_pipe:
> +	close(pipefd[0]);
> +	close(pipefd[1]);
> +}

-- 
Sincerely yours,
Mike.