powerpc/selftests: Check endianness on trap in TM

Add a selftest to check if endianness is flipped inadvertently to BE
(MSR.LE set to zero) on BE and LE machines when a trap is caught in
transactional mode and load_fp and load_vec are zero, i.e. when MSR.FP
and MSR.VEC are zeroed (disabled).
Signed-off-by: Gustavo Romero <gromero@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/tm/.gitignore

@@ -13,3 +13,4 @@ tm-signal-context-chk-vmx
 tm-signal-context-chk-vsx
 tm-vmx-unavail
 tm-unavailable
+tm-trap

tools/testing/selftests/powerpc/tm/Makefile

@@ -3,7 +3,7 @@ SIGNAL_CONTEXT_CHK_TESTS := tm-signal-context-chk-gpr tm-signal-context-chk-fpu
 	tm-signal-context-chk-vmx tm-signal-context-chk-vsx
 
 TEST_GEN_PROGS := tm-resched-dscr tm-syscall tm-signal-msr-resv tm-signal-stack \
-	tm-vmxcopy tm-fork tm-tar tm-tmspr tm-vmx-unavail tm-unavailable \
+	tm-vmxcopy tm-fork tm-tar tm-tmspr tm-vmx-unavail tm-unavailable tm-trap \
 	$(SIGNAL_CONTEXT_CHK_TESTS)
 
 include ../../lib.mk
@@ -18,6 +18,7 @@ $(OUTPUT)/tm-tmspr: CFLAGS += -pthread
 $(OUTPUT)/tm-vmx-unavail: CFLAGS += -pthread -m64
 $(OUTPUT)/tm-resched-dscr: ../pmu/lib.o
 $(OUTPUT)/tm-unavailable: CFLAGS += -O0 -pthread -m64 -Wno-error=uninitialized -mvsx
+$(OUTPUT)/tm-trap: CFLAGS += -O0 -pthread -m64
 
 SIGNAL_CONTEXT_CHK_TESTS := $(patsubst %,$(OUTPUT)/%,$(SIGNAL_CONTEXT_CHK_TESTS))
 $(SIGNAL_CONTEXT_CHK_TESTS): tm-signal.S

tools/testing/selftests/powerpc/tm/tm-trap.c

+/*
+ * Copyright 2017, Gustavo Romero, IBM Corp.
+ * Licensed under GPLv2.
+ *
+ * Check if thread endianness is flipped inadvertently to BE on trap
+ * caught in TM whilst MSR.FP and MSR.VEC are zero (i.e. just after
+ * load_fp and load_vec overflowed).
+ *
+ * The issue can be checked on LE machines simply by zeroing load_fp
+ * and load_vec and then causing a trap in TM. Since the endianness
+ * changes to BE on return from the signal handler, 'nop' is
+ * thread as an illegal instruction in following sequence:
+ *	tbegin.
+ *	beq 1f
+ *	trap
+ *	tend.
+ * 1:	nop
+ *
+ * However, although the issue is also present on BE machines, it's a
+ * bit trickier to check it on BE machines because MSR.LE bit is set
+ * to zero which determines a BE endianness that is the native
+ * endianness on BE machines, so nothing notably critical happens,
+ * i.e. no illegal instruction is observed immediately after returning
+ * from the signal handler (as it happens on LE machines). Thus to test
+ * it on BE machines LE endianness is forced after a first trap and then
+ * the endianness is verified on subsequent traps to determine if the
+ * endianness "flipped back" to the native endianness (BE).
+ */
+
+#define _GNU_SOURCE
+#include <error.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <htmintrin.h>
+#include <inttypes.h>
+#include <pthread.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdbool.h>
+
+#include "tm.h"
+#include "utils.h"
+
+#define pr_error(error_code, format, ...) \
+	error_at_line(1, error_code, __FILE__, __LINE__, format, ##__VA_ARGS__)
+
+#define MSR_LE 1UL
+#define LE     1UL
+
+pthread_t t0_ping;
+pthread_t t1_pong;
+
+int exit_from_pong;
+
+int trap_event;
+int le;
+
+bool success;
+
+void trap_signal_handler(int signo, siginfo_t *si, void *uc)
+{
+	ucontext_t *ucp = uc;
+	uint64_t thread_endianness;
+
+	/* Get thread endianness: extract bit LE from MSR */
+	thread_endianness = MSR_LE & ucp->uc_mcontext.gp_regs[PT_MSR];
+
+	/***
+	 * Little-Endian Machine
+	 */
+
+	if (le) {
+		/* First trap event */
+		if (trap_event == 0) {
+			/* Do nothing. Since it is returning from this trap
+			 * event that endianness is flipped by the bug, so just
+			 * let the process return from the signal handler and
+			 * check on the second trap event if endianness is
+			 * flipped or not.
+			 */
+		}
+		/* Second trap event */
+		else if (trap_event == 1) {
+			/*
+			 * Since trap was caught in TM on first trap event, if
+			 * endianness was still LE (not flipped inadvertently)
+			 * after returning from the signal handler instruction
+			 * (1) is executed (basically a 'nop'), as it's located
+			 * at address of tbegin. +4 (rollback addr). As (1) on
+			 * LE endianness does in effect nothing, instruction (2)
+			 * is then executed again as 'trap', generating a second
+			 * trap event (note that in that case 'trap' is caught
+			 * not in transacional mode). On te other hand, if after
+			 * the return from the signal handler the endianness in-
+			 * advertently flipped, instruction (1) is tread as a
+			 * branch instruction, i.e. b .+8, hence instruction (3)
+			 * and (4) are executed (tbegin.; trap;) and we get sim-
+			 * ilaly on the trap signal handler, but now in TM mode.
+			 * Either way, it's now possible to check the MSR LE bit
+			 * once in the trap handler to verify if endianness was
+			 * flipped or not after the return from the second trap
+			 * event. If endianness is flipped, the bug is present.
+			 * Finally, getting a trap in TM mode or not is just
+			 * worth noting because it affects the math to determine
+			 * the offset added to the NIP on return: the NIP for a
+			 * trap caught in TM is the rollback address, i.e. the
+			 * next instruction after 'tbegin.', whilst the NIP for
+			 * a trap caught in non-transactional mode is the very
+			 * same address of the 'trap' instruction that generated
+			 * the trap event.
+			 */
+
+			if (thread_endianness == LE) {
+				/* Go to 'success', i.e. instruction (6) */
+				ucp->uc_mcontext.gp_regs[PT_NIP] += 16;
+			} else {
+				/*
+				 * Thread endianness is BE, so it flipped
+				 * inadvertently. Thus we flip back to LE and
+				 * set NIP to go to 'failure', instruction (5).
+				 */
+				ucp->uc_mcontext.gp_regs[PT_MSR] |= 1UL;
+				ucp->uc_mcontext.gp_regs[PT_NIP] += 4;
+			}
+		}
+	}
+
+	/***
+	 * Big-Endian Machine
+	 */
+
+	else {
+		/* First trap event */
+		if (trap_event == 0) {
+			/*
+			 * Force thread endianness to be LE. Instructions (1),
+			 * (3), and (4) will be executed, generating a second
+			 * trap in TM mode.
+			 */
+			ucp->uc_mcontext.gp_regs[PT_MSR] |= 1UL;
+		}
+		/* Second trap event */
+		else if (trap_event == 1) {
+			/*
+			 * Do nothing. If bug is present on return from this
+			 * second trap event endianness will flip back "automat-
+			 * ically" to BE, otherwise thread endianness will
+			 * continue to be LE, just as it was set above.
+			 */
+		}
+		/* A third trap event */
+		else {
+			/*
+			 * Once here it means that after returning from the sec-
+			 * ond trap event instruction (4) (trap) was executed
+			 * as LE, generating a third trap event. In that case
+			 * endianness is still LE as set on return from the
+			 * first trap event, hence no bug. Otherwise, bug
+			 * flipped back to BE on return from the second trap
+			 * event and instruction (4) was executed as 'tdi' (so
+			 * basically a 'nop') and branch to 'failure' in
+			 * instruction (5) was taken to indicate failure and we
+			 * never get here.
+			 */
+
+			/*
+			 * Flip back to BE and go to instruction (6), i.e. go to
+			 * 'success'.
+			 */
+			ucp->uc_mcontext.gp_regs[PT_MSR] &= ~1UL;
+			ucp->uc_mcontext.gp_regs[PT_NIP] += 8;
+		}
+	}
+
+	trap_event++;
+}
+
+void usr1_signal_handler(int signo, siginfo_t *si, void *not_used)
+{
+	/* Got a USR1 signal from ping(), so just tell pong() to exit */
+	exit_from_pong = 1;
+}
+
+void *ping(void *not_used)
+{
+	uint64_t i;
+
+	trap_event = 0;
+
+	/*
+	 * Wait an amount of context switches so load_fp and load_vec overflows
+	 * and MSR_[FP|VEC|V] is 0.
+	 */
+	for (i = 0; i < 1024*1024*512; i++)
+		;
+
+	asm goto(
+		/*
+		 * [NA] means "Native Endianness", i.e. it tells how a
+		 * instruction is executed on machine's native endianness (in
+		 * other words, native endianness matches kernel endianness).
+		 * [OP] means "Opposite Endianness", i.e. on a BE machine, it
+		 * tells how a instruction is executed as a LE instruction; con-
+		 * versely, on a LE machine, it tells how a instruction is
+		 * executed as a BE instruction. When [NA] is omitted, it means
+		 * that the native interpretation of a given instruction is not
+		 * relevant for the test. Likewise when [OP] is omitted.
+		 */
+
+		" tbegin.        ;" /* (0) tbegin. [NA]                    */
+		" tdi  0, 0, 0x48;" /* (1) nop     [NA]; b (3) [OP]        */
+		" trap           ;" /* (2) trap    [NA]                    */
+		".long 0x1D05007C;" /* (3) tbegin. [OP]                    */
+		".long 0x0800E07F;" /* (4) trap    [OP]; nop   [NA]        */
+		" b %l[failure]  ;" /* (5) b [NA]; MSR.LE flipped (bug)    */
+		" b %l[success]  ;" /* (6) b [NA]; MSR.LE did not flip (ok)*/
+
+		: : : : failure, success);
+
+failure:
+	success = false;
+	goto exit_from_ping;
+
+success:
+	success = true;
+
+exit_from_ping:
+	/* Tell pong() to exit before leaving */
+	pthread_kill(t1_pong, SIGUSR1);
+	return NULL;
+}
+
+void *pong(void *not_used)
+{
+	while (!exit_from_pong)
+		/*
+		 * Induce context switches on ping() thread
+		 * until ping() finishes its job and signs
+		 * to exit from this loop.
+		 */
+		sched_yield();
+
+	return NULL;
+}
+
+int tm_trap_test(void)
+{
+	uint16_t k = 1;
+
+	int rc;
+
+	pthread_attr_t attr;
+	cpu_set_t cpuset;
+
+	struct sigaction trap_sa;
+
+	trap_sa.sa_flags = SA_SIGINFO;
+	trap_sa.sa_sigaction = trap_signal_handler;
+	sigaction(SIGTRAP, &trap_sa, NULL);
+
+	struct sigaction usr1_sa;
+
+	usr1_sa.sa_flags = SA_SIGINFO;
+	usr1_sa.sa_sigaction = usr1_signal_handler;
+	sigaction(SIGUSR1, &usr1_sa, NULL);
+
+	/* Set only CPU 0 in the mask. Both threads will be bound to cpu 0. */
+	CPU_ZERO(&cpuset);
+	CPU_SET(0, &cpuset);
+
+	/* Init pthread attribute */
+	rc = pthread_attr_init(&attr);
+	if (rc)
+		pr_error(rc, "pthread_attr_init()");
+
+	/*
+	 * Bind thread ping() and pong() both to CPU 0 so they ping-pong and
+	 * speed up context switches on ping() thread, speeding up the load_fp
+	 * and load_vec overflow.
+	 */
+	rc = pthread_attr_setaffinity_np(&attr, sizeof(cpu_set_t), &cpuset);
+	if (rc)
+		pr_error(rc, "pthread_attr_setaffinity()");
+
+	/* Figure out the machine endianness */
+	le = (int) *(uint8_t *)&k;
+
+	printf("%s machine detected. Checking if endianness flips %s",
+		le ? "Little-Endian" : "Big-Endian",
+		"inadvertently on trap in TM... ");
+
+	rc = fflush(0);
+	if (rc)
+		pr_error(rc, "fflush()");
+
+	/* Launch ping() */
+	rc = pthread_create(&t0_ping, &attr, ping, NULL);
+	if (rc)
+		pr_error(rc, "pthread_create()");
+
+	exit_from_pong = 0;
+
+	/* Launch pong() */
+	rc = pthread_create(&t1_pong, &attr, pong, NULL);
+	if (rc)
+		pr_error(rc, "pthread_create()");
+
+	rc = pthread_join(t0_ping, NULL);
+	if (rc)
+		pr_error(rc, "pthread_join()");
+
+	rc = pthread_join(t1_pong, NULL);
+	if (rc)
+		pr_error(rc, "pthread_join()");
+
+	if (success) {
+		printf("no.\n"); /* no, endianness did not flip inadvertently */
+		return EXIT_SUCCESS;
+	}
+
+	printf("yes!\n"); /* yes, endianness did flip inadvertently */
+	return EXIT_FAILURE;
+}
+
+int main(int argc, char **argv)
+{
+	return test_harness(tm_trap_test, "tm_trap_test");
+}