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

On 2019-10-12, Michael Ellerman [off-list ref] wrote:

Aleksa Sarai [off-list ref] writes:

quoted

On 2019-10-11, Michael Ellerman [off-list ref] wrote:

quoted

On a machine with a 64K PAGE_SIZE, the nested for loops in
test_check_nonzero_user() can lead to soft lockups, eg:

...

quoted

quoted

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.

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