Re: [PATCH] usercopy: Avoid soft lockups in test_check_nonzero_user()

[Aleksa Sarai]

On 2019-10-12, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> Aleksa Sarai <cyphar@xxxxxxxxxx> writes:
> > On 2019-10-11, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> >> On a machine with a 64K PAGE_SIZE, the nested for loops in
> >> test_check_nonzero_user() can lead to soft lockups, eg:
> ...
> >> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> >> index 950ee88cd6ac..9fb6bc609d4c 100644
> >> --- a/lib/test_user_copy.c
> >> +++ b/lib/test_user_copy.c
> >> @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size)
> >>  static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size)
> >>  {
> >>  	int ret = 0;
> >> -	size_t start, end, i;
> >> -	size_t zero_start = size / 4;
> >> -	size_t zero_end = size - zero_start;
> >> +	size_t start, end, i, zero_start, zero_end;
> >> +
> >> +	if (test(size < 1024, "buffer too small"))
> >> +		return -EINVAL;
> >> +
> >> +	/*
> >> +	 * We want to cross a page boundary to exercise the code more
> >> +	 * effectively. We assume the buffer we're passed has a page boundary at
> >> +	 * size / 2. We also don't want to make the size we scan too large,
> >> +	 * otherwise the test can take a long time and cause soft lockups. So
> >> +	 * scan a 1024 byte region across the page boundary.
> >> +	 */
> >> +	start = size / 2 - 512;
> >> +	size = 1024;
> >
> > I don't think it's necessary to do "size / 2" here -- you can just use
> > PAGE_SIZE directly and check above that "size == 2*PAGE_SIZE" (not that
> > this check is exceptionally necessary -- since there's only one caller
> > of this function and it's in the same file).
> 
> OK, like this?

Yup -- that looks good. I'll give it a Reviewed-by once you resend it.

> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 950ee88cd6ac..48bc669b2549 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -47,9 +47,25 @@ static bool is_zeroed(void *from, size_t size)
>  static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size)
>  {
>  	int ret = 0;
> -	size_t start, end, i;
> -	size_t zero_start = size / 4;
> -	size_t zero_end = size - zero_start;
> +	size_t start, end, i, zero_start, zero_end;
> +
> +	if (test(size < 2 * PAGE_SIZE, "buffer too small"))
> +		return -EINVAL;
> +
> +	/*
> +	 * We want to cross a page boundary to exercise the code more
> +	 * effectively. We also don't want to make the size we scan too large,
> +	 * otherwise the test can take a long time and cause soft lockups. So
> +	 * scan a 1024 byte region across the page boundary.
> +	 */
> +	size = 1024;
> +	start = PAGE_SIZE - (size / 2);
> +
> +	kmem += start;
> +	umem += start;
> +
> +	zero_start = size / 4;
> +	zero_end = size - zero_start;
>  
>  	/*
>  	 * We conduct a series of check_nonzero_user() tests on a block of memory


-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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