[PATCH 1/7] KVM: selftests: Account for kernel's off-by-one bug in NUMA node syscalls

[Sean Christopherson <seanjc@google.com>]

Add and use MAXNODE_FOR_MASK() to compute the "correct" maxnode value that
is passed to various NUMA-related syscalls, e.g. get_mempolicy(), mbind(),
migrate_pages(), etc.  In quotes, because the kernel has an undocumented,
longstanding off-by-one bug that requires userspace to specify the number
of bits plus one, i.e. the max node plus two.

The kernel bug has been known since 2007[*]:

 : And this is I think the reason why we can't change this now. I assume
 : numactl allocates 1024 bits (0 to 1023) and passes 1025 to make sure all
 : 1024 bits are processed. If we change it now, kernel will process 1025
 : bits (0 to 1024) and overflow the allocated bitmask. If it happens to be
 : at the border of mmaped vma, it's a segfault...

But unfortunately the manpages haven't yet been updated, e.g.

 : The maxnode argument is the maximum node number in the bit mask plus one

Link: https://lore.kernel.org/all/63ccc890-fd57-118b-5997-e0259f507d28@suse.cz[*]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 tools/testing/selftests/kvm/guest_memfd_test.c   |  2 +-
 tools/testing/selftests/kvm/include/numaif.h     | 11 +++++++++++
 tools/testing/selftests/kvm/x86/xapic_ipi_test.c |  2 +-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index 2233d871a38f..cd5df88bc642 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -80,7 +80,7 @@ static void test_mbind(int fd, size_t total_size)
 {
 	const unsigned long nodemask_0 = 1; /* nid: 0 */
 	unsigned long nodemask = 0;
-	unsigned long maxnode = BITS_PER_TYPE(nodemask);
+	unsigned long maxnode = MAXNODE_FOR_MASK(nodemask);
 	int policy;
 	char *mem;
 	int ret;
diff --git a/tools/testing/selftests/kvm/include/numaif.h b/tools/testing/selftests/kvm/include/numaif.h
index 29572a6d789c..71f261eafc90 100644
--- a/tools/testing/selftests/kvm/include/numaif.h
+++ b/tools/testing/selftests/kvm/include/numaif.h
@@ -6,6 +6,7 @@
 
 #include <dirent.h>
 
+#include <linux/bitops.h>
 #include <linux/mempolicy.h>
 
 #include "kvm_syscalls.h"
@@ -30,6 +31,16 @@ KVM_SYSCALL_DEFINE(mbind, 6, void *, addr, unsigned long, size, int, mode,
 		   const unsigned long *, nodemask, unsigned long, maxnode,
 		   unsigned int, flags);
 
+/*
+ * Calculate the @maxnode param for the above syscalls given the mask that will
+ * be passed to the kernel, to account for a longstanding off-by-one bug in the
+ * kernel that isn't properly documented in the manpages.  The manpages say
+ * that @maxnode is "the maximum node ID plus one", but the kernel's actual
+ * behavior is "the number of bits in the mask plus one", i.e. "the maximum
+ * node ID plus two".
+ */
+#define MAXNODE_FOR_MASK(mask) (BITS_PER_TYPE(mask) + 1)
+
 static inline int get_max_numa_node(void)
 {
 	struct dirent *de;
diff --git a/tools/testing/selftests/kvm/x86/xapic_ipi_test.c b/tools/testing/selftests/kvm/x86/xapic_ipi_test.c
index 469e3ab16460..1ddcf95d7fe4 100644
--- a/tools/testing/selftests/kvm/x86/xapic_ipi_test.c
+++ b/tools/testing/selftests/kvm/x86/xapic_ipi_test.c
@@ -248,7 +248,7 @@ void do_migrations(struct test_data_page *data, int run_secs, int delay_usecs,
 		delay_usecs);
 
 	/* Get set of first 64 numa nodes available */
-	kvm_get_mempolicy(NULL, &nodemask, sizeof(nodemask) * 8,
+	kvm_get_mempolicy(NULL, &nodemask, MAXNODE_FOR_MASK(nodemask),
 			  0, MPOL_F_MEMS_ALLOWED);
 
 	fprintf(stderr, "Numa nodes found amongst first %lu possible nodes "
-- 
2.55.0.970.g62bdec98f9-goog