Fix mempolicy_test on bazel.

Bazel runs multiple test cases on the same thread. Some of the test
cases rely on the test thread starting with the default memory policy,
while other tests modify the test thread's memory policy. This
obviously breaks when the test framework doesn't run each test case on
a new thread.

Also fixing an incompatibility where set_mempolicy(2) was prevented
from specifying an empty nodemask, which is allowed for some modes.

PiperOrigin-RevId: 224038957
Change-Id: Ibf780766f2706ebc9b129dbc8cf1b85c2a275074

1 files changed, 10 insertions, 7 deletions

diff --git a/pkg/sentry/syscalls/linux/sys_mmap.go b/pkg/sentry/syscalls/linux/sys_mmap.go
index f8d9c43fd..145f7846c 100644
--- a/pkg/sentry/syscalls/linux/sys_mmap.go
+++ b/pkg/sentry/syscalls/linux/sys_mmap.go
@@ -308,28 +308,31 @@ func SetMempolicy(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.
 	nodemask := args[1].Pointer()
 	maxnode := args[2].Uint()
 
-	if maxnode < 1 {
+	if nodemask != 0 && maxnode < 1 {
 		return 0, nil, syserror.EINVAL
 	}
 
 	if modeWithFlags&linux.MPOL_MODE_FLAGS == linux.MPOL_MODE_FLAGS {
-		// Can't specify multiple modes simultaneously. Must also contain a
-		// valid mode, which we check below.
+		// Can't specify multiple modes simultaneously.
 		return 0, nil, syserror.EINVAL
 	}
 
 	mode := modeWithFlags &^ linux.MPOL_MODE_FLAGS
 	if mode < 0 || mode >= linux.MPOL_MAX {
+		// Must specify a valid mode.
 		return 0, nil, syserror.EINVAL
 	}
 
 	var nodemaskVal uint32
-	if _, err := t.CopyIn(nodemask, &nodemaskVal); err != nil {
-		return 0, nil, syserror.EFAULT
+	// Nodemask may be empty for some policy modes.
+	if nodemask != 0 && maxnode > 0 {
+		if _, err := t.CopyIn(nodemask, &nodemaskVal); err != nil {
+			return 0, nil, syserror.EFAULT
+		}
 	}
 
-	// When setting MPOL_INTERLEAVE, nodemask must not be empty.
-	if mode == linux.MPOL_INTERLEAVE && nodemaskVal == 0 {
+	if (mode == linux.MPOL_INTERLEAVE || mode == linux.MPOL_BIND) && nodemaskVal == 0 {
+		// Mode requires a non-empty nodemask, but got an empty nodemask.
 		return 0, nil, syserror.EINVAL
 	}