new file mode 100644
@@ -0,0 +1,80 @@
+/*
+ * Test patching code, running in one thread, from another thread.
+ *
+ * Intel SDM calls this "cross-modifying code" and recommends a special
+ * sequence, which requires both threads to cooperate.
+ *
+ * Linux kernel uses a different sequence that does not require cooperation and
+ * involves patching the first byte with int3.
+ *
+ * Finally, there is user-mode software out there that simply uses atomics, and
+ * that seems to be good enough in practice. Test that QEMU has no problems
+ * with this as well.
+ */
+
+#include <assert.h>
+#include <pthread.h>
+#include <stdbool.h>
+#include <stdlib.h>
+
+void add1_or_nop(long *x);
+asm(".pushsection .rwx,\"awx\",@progbits\n"
+ ".globl add1_or_nop\n"
+ /* addq $0x1,(%rdi) */
+ "add1_or_nop: .byte 0x48, 0x83, 0x07, 0x01\n"
+ "ret\n"
+ ".popsection\n");
+
+#define THREAD_WAIT 0
+#define THREAD_PATCH 1
+#define THREAD_STOP 2
+
+static void *thread_func(void *arg)
+{
+ int val = 0x0026748d; /* nop */
+
+ while (true) {
+ switch (__atomic_load_n((int *)arg, __ATOMIC_SEQ_CST)) {
+ case THREAD_WAIT:
+ break;
+ case THREAD_PATCH:
+ val = __atomic_exchange_n((int *)&add1_or_nop, val,
+ __ATOMIC_SEQ_CST);
+ break;
+ case THREAD_STOP:
+ return NULL;
+ default:
+ assert(false);
+ __builtin_unreachable();
+ }
+ }
+}
+
+#define INITIAL 42
+#define COUNT 1000000
+
+int main(void)
+{
+ int command = THREAD_WAIT;
+ pthread_t thread;
+ long x = 0;
+ int err;
+ int i;
+
+ err = pthread_create(&thread, NULL, &thread_func, &command);
+ assert(err == 0);
+
+ __atomic_store_n(&command, THREAD_PATCH, __ATOMIC_SEQ_CST);
+ for (i = 0; i < COUNT; i++) {
+ add1_or_nop(&x);
+ }
+ __atomic_store_n(&command, THREAD_STOP, __ATOMIC_SEQ_CST);
+
+ err = pthread_join(thread, NULL);
+ assert(err == 0);
+
+ assert(x >= INITIAL);
+ assert(x <= INITIAL + COUNT);
+
+ return EXIT_SUCCESS;
+}
@@ -17,6 +17,7 @@ X86_64_TESTS += cmpxchg
X86_64_TESTS += adox
X86_64_TESTS += test-1648
X86_64_TESTS += test-2175
+X86_64_TESTS += cross-modifying-code
TESTS=$(MULTIARCH_TESTS) $(X86_64_TESTS) test-x86_64
else
TESTS=$(MULTIARCH_TESTS)
@@ -27,6 +28,9 @@ adox: CFLAGS=-O2
run-test-i386-ssse3: QEMU_OPTS += -cpu max
run-plugin-test-i386-ssse3-%: QEMU_OPTS += -cpu max
+cross-modifying-code: CFLAGS+=-pthread
+cross-modifying-code: LDFLAGS+=-pthread
+
test-x86_64: LDFLAGS+=-lm -lc
test-x86_64: test-i386.c test-i386.h test-i386-shift.h test-i386-muldiv.h
$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)