[PATCH v5 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass

[Wilson Felipe Pereira <wfelipe@google.com>]

In test_no_kmem_bypass(), delta (stored_pages * page_size - zswapped) is
checked against stored_pages * page_size / 4 to verify that the pages
pushed to zswap belong to the test memory cgroup.

Due to slight stat update timing differences, delta can evaluate to a small
negative number (e.g. -5MB out of 1GB). Because delta is declared as a
signed int and stored_pages is an unsigned size_t, C's usual arithmetic
conversions implicitly promote a negative delta to a large unsigned 64-bit
integer, causing `delta < stored_pages * page_size / 4` to falsely evaluate
to 0 and fail the test.

Fix this by declaring zswapped and delta as signed long long and comparing
against a signed threshold, ensuring negative deltas correctly evaluate
to true.

Fixes: a549f9f31561a ("selftests: cgroup: add test_zswap with no kmem bypass test")
Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
Acked-by: Michal Koutný <mkoutny@suse.com>
---
 tools/testing/selftests/cgroup/lib/cgroup_util.c    |  9 +++++++--
 .../selftests/cgroup/lib/include/cgroup_util.h      |  1 +
 tools/testing/selftests/cgroup/test_zswap.c         | 13 ++++++++-----
 3 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/cgroup/lib/cgroup_util.c b/tools/testing/selftests/cgroup/lib/cgroup_util.c
index 2596c12cd864..ce2dd36291f8 100644
--- a/tools/testing/selftests/cgroup/lib/cgroup_util.c
+++ b/tools/testing/selftests/cgroup/lib/cgroup_util.c
@@ -172,7 +172,7 @@ long cg_read_long_fd(int fd)
 	return atol(buf);
 }
 
-long cg_read_key_long(const char *cgroup, const char *control, const char *key)
+long long cg_read_key_long_long(const char *cgroup, const char *control, const char *key)
 {
 	char buf[BUF_SIZE];
 	char *ptr;
@@ -184,7 +184,12 @@ long cg_read_key_long(const char *cgroup, const char *control, const char *key)
 	if (!ptr)
 		return -1;
 
-	return atol(ptr + strlen(key));
+	return atoll(ptr + strlen(key));
+}
+
+long cg_read_key_long(const char *cgroup, const char *control, const char *key)
+{
+	return (long)cg_read_key_long_long(cgroup, control, key);
 }
 
 long cg_read_key_long_poll(const char *cgroup, const char *control,
diff --git a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
index 5d39c709ac7a..13923534003c 100644
--- a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
+++ b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
@@ -69,6 +69,7 @@ extern int cg_read_strstr(const char *cgroup, const char *control,
 			  const char *needle);
 extern long cg_read_long(const char *cgroup, const char *control);
 extern long cg_read_long_fd(int fd);
+long long cg_read_key_long_long(const char *cgroup, const char *control, const char *key);
 long cg_read_key_long(const char *cgroup, const char *control, const char *key);
 long cg_read_key_long_poll(const char *cgroup, const char *control,
 			   const char *key, long expected, int retries,
diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 8f2c9aa4776c..11c67dde705f 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -630,11 +630,14 @@ static int test_no_kmem_bypass(const char *root)
 			break;
 		/* If memory was pushed to zswap, verify it belongs to memcg */
 		if (stored_pages > stored_pages_threshold) {
-			int zswapped = cg_read_key_long(test_group, "memory.stat", "zswapped ");
-			int delta = stored_pages * page_size - zswapped;
-			int result_ok = delta < stored_pages * page_size / 4;
-
-			ret = result_ok ? KSFT_PASS : KSFT_FAIL;
+			long long zswapped = cg_read_key_long_long(
+				test_group, "memory.stat", "zswapped ");
+			long long delta =
+				(long long)stored_pages * page_size - zswapped;
+			long long max_delta =
+				(long long)stored_pages * page_size / 4;
+
+			ret = (delta < max_delta) ? KSFT_PASS : KSFT_FAIL;
 			break;
 		}
 	}
-- 
2.55.0.970.g62bdec98f9-goog