[RFC PATCH 16/16] x86/split_lock: Add user space split lock test in selftest

From: Fenghua Yu
Date: Sun May 27 2018 - 11:47:35 EST


User may use the selftest to test how user space split lock is handled,

If #AC exception is enabled for split lock, the test generates split
locked access from user space and tests how the split lock access is
handled in two ways:

1. A SIGBUS is delivered to the test processor. This is specified by
writing "sigbus" to /sys/kernel/debug/x86/split_lock/user_mode.

2. The faulting instruction is re-executed. This is specified by
writing "re-execute" to /sys/kernel/debug/x86/split_lock/user_mode.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
tools/testing/selftests/x86/Makefile | 3 +-
tools/testing/selftests/x86/split_lock_user_test.c | 207 +++++++++++++++++++++
2 files changed, 209 insertions(+), 1 deletion(-)
create mode 100644 tools/testing/selftests/x86/split_lock_user_test.c

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 39f66bc29b82..e4bcb72c0ae8 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -11,7 +11,8 @@ CAN_BUILD_X86_64 := $(shell ./check_cc.sh $(CC) trivial_64bit_program.c)

TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
check_initial_reg_state sigreturn iopl mpx-mini-test ioperm \
- protection_keys test_vdso test_vsyscall mov_ss_trap
+ protection_keys test_vdso test_vsyscall mov_ss_trap \
+ split_lock_user_test
TARGETS_C_32BIT_ONLY := entry_from_vm86 syscall_arg_fault test_syscall_vdso unwind_vdso \
test_FCMOV test_FCOMI test_FISTTP \
vdso_restorer
diff --git a/tools/testing/selftests/x86/split_lock_user_test.c b/tools/testing/selftests/x86/split_lock_user_test.c
new file mode 100644
index 000000000000..9ce5fc1af154
--- /dev/null
+++ b/tools/testing/selftests/x86/split_lock_user_test.c
@@ -0,0 +1,207 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2018 Intel Corporation
+ * Author: Fenghua Yu <fenghua.yu@xxxxxxxxx>
+ *
+ * Pre-request:
+ * Kernel is built with CONFIG_SPLIT_LOCK_AC=y.
+ * Split lock is enabled. If not, enable it by:
+ * #echo 1 >/sys/kernel/debug/x86/split_lock/enable
+ *
+ * Usage:
+ * Run the test alone and it should show:
+ * TEST PASS: locked instruction is re-executed.
+ * TEST PASS: Caught SIGBUS/#AC due to split locked access
+ *
+ * Or launch the test from perf and watch "split_lock_user" event count.
+ * #/perf stat -e sq_misc.split_lock /root/split_lock_user_test_64
+ * TEST PASS: locked instruction is re-executed.
+ * TEST PASS: Caught SIGBUS/#AC due to split locked access
+ *
+ * Performance counter stats for 'tools/testing/selftests/x86/
+ * split_lock_user_test_64':
+ * 2 sq_misc.split_lock
+ *
+ * 1.001507372 seconds time elapsed
+ */
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+
+int split_lock_exception;
+
+void catch_sigbus(int sig)
+{
+ split_lock_exception = 1;
+ printf("TEST PASS: Caught SIGBUS/#AC due to split locked access\n");
+
+ exit(-1);
+}
+
+int split_lock_ac_enabled(void)
+{
+ int fd, enable, ret;
+ char buf[16];
+
+ fd = open("/sys/kernel/debug/x86/split_lock/enable", O_RDONLY);
+ if (fd < 0)
+ return 0;
+
+ if (read(fd, buf, sizeof(int)) < 0) {
+ ret = 0;
+ goto out;
+ }
+
+ enable = atoi(buf);
+ if (enable == 1)
+ ret = 1;
+ else
+ ret = 0;
+
+out:
+ close(fd);
+
+ return ret;
+}
+
+int setup_user_mode(char *user_mode_reaction)
+{
+ ssize_t count;
+ int fd;
+
+ if (strcmp(user_mode_reaction, "sigbus") &&
+ strcmp(user_mode_reaction, "re-execute"))
+ return -1;
+
+ fd = open("/sys/kernel/debug/x86/split_lock/user_mode", O_RDWR);
+ if (fd < 0)
+ return -1;
+
+ count = write(fd, user_mode_reaction, strlen(user_mode_reaction));
+ if (count != strlen(user_mode_reaction))
+ return -1;
+
+ close(fd);
+
+ return 0;
+}
+
+/*
+ * Atomically add 1 to *iptr using "lock addl" instruction.
+ * Since *iptr crosses two cache lines, #AC is generated by the split lock.
+ */
+void do_split_locked_inst(int *iptr)
+{
+ /*
+ * The distance between iptr and next cache line is 3 bytes.
+ * Operand size in "addl" is 4 bytes. So iptr will span two cache
+ * lines. "lock addl" instruction will trigger #AC in hardware
+ * and kernel either delivers SIGBUS to this process or re-execute
+ * the instruction depending on
+ * /sys/kernel/debug/x86/split_lock/user_mode setting.
+ */
+ asm volatile ("lock addl $1, %0\n\t"
+ : "=m" (*iptr));
+}
+
+/*
+ * Test re-executing a locked instruction after it generates #AC for split lock
+ * operand *iptr.
+ */
+void test_re_execute(int *iptr)
+{
+ setup_user_mode("re-execute");
+
+ /* Initialize *iptr. */
+ *iptr = 0;
+
+ /* The locked instruction triggers #AC and then it's re-executed. */
+ do_split_locked_inst(iptr);
+
+ /* If executed successfully, *iptr should be 1 now. */
+ if (*iptr == 1) {
+ printf("TEST PASS: locked instruction is re-executed.\n");
+ } else {
+ printf("TEST FAIL: No #AC exception is caught and ");
+ printf("instruction is not executed correctly.\n");
+ }
+}
+
+/*
+ * Test SIGBUS delivered after a lock instruction generates #AC for split lock
+ * operand *iptr.
+ */
+void test_sigbus(int *iptr)
+{
+ pid_t pid;
+
+ setup_user_mode("sigbus");
+
+ pid = fork();
+ if (pid) {
+ waitpid(pid, NULL, WIFSIGNALED(NULL));
+ return;
+ }
+
+ /*
+ * The locked instruction will trigger #AC and kernel will deliver
+ * SIGBUS to this process. The SIGBUS handler in this process will
+ * verify that the signal is delivered and the process is killed then.
+ */
+ do_split_locked_inst(iptr);
+}
+
+int main(int argc, char **argv)
+{
+ int *iptr;
+ char *cptr;
+
+ if (!split_lock_ac_enabled()) {
+ printf("#AC exception for split lock is NOT enabled!!\n");
+ printf("Before test, please make sure:\n");
+ printf("split lock feature is supported on this platform,\n");
+ printf("CONFIG_SPLIT_LOCK_AC is turned on,\n");
+ printf("and /sys/kernel/debug/x86/split_lock/enable is 1.\n");
+
+ return 0;
+ }
+
+ signal(SIGBUS, catch_sigbus);
+
+ /*
+ * Enable Alignment Checking on x86_64.
+ * This will generate alignment check on not only split lock but also
+ * on any misalignment.
+ * Turn on this for reference only.
+ */
+ /* __asm__("pushf\norl $0x40000,(%rsp)\npopf"); */
+
+ /* aligned_alloc() provides 64-byte aligned memory */
+ cptr = (char *)aligned_alloc(64, 128);
+
+ /*
+ * Increment the pointer by 61, making it 3 bytes away from the next
+ * cache line and 4-byte *iptr across two cache line.
+ */
+ iptr = (int *)(cptr + 61);
+
+ test_re_execute(iptr);
+ /*
+ * The split lock is disabled after the last locked instruction is
+ * re-executed.
+ *
+ * Wait for the split lock is re-enabled again before next test.
+ */
+ sleep(1);
+ test_sigbus(iptr);
+
+ free(cptr);
+
+ return 0;
+}
--
2.5.0