selftests/powerpc: Add tm-signal-pagefault test

This test triggers a TM Bad Thing by raising a signal in transactional state
and forcing a pagefault to happen in kernelspace when the kernel signal
handling code first touches the user signal stack.

This is inspired by the test tm-signal-context-force-tm but uses userfaultfd to
make the test deterministic. While this test always triggers the bug in one
run, I had to execute tm-signal-context-force-tm several times (the test runs
5000 times each execution) to trigger the same bug.

tm-signal-context-force-tm is kept instead of replaced because, while this test
is more reliable and triggers the same bug, tm-signal-context-force-tm has a
better coverage, in the sense that by running the test several times it might
trigger the pagefault and/or be preempted at different places.

v3: skip test if userfaultfd is unavailable.
Signed-off-by: Gustavo Luiz Duarte <gustavold@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200211033831.11165-2-gustavold@linux.ibm.com

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

@@ -13,6 +13,7 @@ tm-signal-context-chk-vmx
 tm-signal-context-chk-vsx
 tm-signal-context-force-tm
 tm-signal-sigreturn-nt
+tm-signal-pagefault
 tm-vmx-unavail
 tm-unavailable
 tm-trap

tools/testing/selftests/powerpc/tm/Makefile

@@ -5,7 +5,7 @@ SIGNAL_CONTEXT_CHK_TESTS := tm-signal-context-chk-gpr tm-signal-context-chk-fpu
 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-trap \
 	$(SIGNAL_CONTEXT_CHK_TESTS) tm-sigreturn tm-signal-sigreturn-nt \
-	tm-signal-context-force-tm tm-poison
+	tm-signal-context-force-tm tm-poison tm-signal-pagefault
 
 top_srcdir = ../../../../..
 include ../../lib.mk
@@ -22,6 +22,7 @@ $(OUTPUT)/tm-resched-dscr: ../pmu/lib.c
 $(OUTPUT)/tm-unavailable: CFLAGS += -O0 -pthread -m64 -Wno-error=uninitialized -mvsx
 $(OUTPUT)/tm-trap: CFLAGS += -O0 -pthread -m64
 $(OUTPUT)/tm-signal-context-force-tm: CFLAGS += -pthread -m64
+$(OUTPUT)/tm-signal-pagefault: CFLAGS += -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-signal-pagefault.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright 2020, Gustavo Luiz Duarte, IBM Corp.
+ *
+ * This test starts a transaction and triggers a signal, forcing a pagefault to
+ * happen when the kernel signal handling code touches the user signal stack.
+ *
+ * In order to avoid pre-faulting the signal stack memory and to force the
+ * pagefault to happen precisely in the kernel signal handling code, the
+ * pagefault handling is done in userspace using the userfaultfd facility.
+ *
+ * Further pagefaults are triggered by crafting the signal handler's ucontext
+ * to point to additional memory regions managed by the userfaultfd, so using
+ * the same mechanism used to avoid pre-faulting the signal stack memory.
+ *
+ * On failure (bug is present) kernel crashes or never returns control back to
+ * userspace. If bug is not present, tests completes almost immediately.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/userfaultfd.h>
+#include <poll.h>
+#include <unistd.h>
+#include <sys/ioctl.h>
+#include <sys/syscall.h>
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <pthread.h>
+#include <signal.h>
+#include <errno.h>
+
+#include "tm.h"
+
+
+#define UF_MEM_SIZE 655360	/* 10 x 64k pages */
+
+/* Memory handled by userfaultfd */
+static char *uf_mem;
+static size_t uf_mem_offset = 0;
+
+/*
+ * Data that will be copied into the faulting pages (instead of zero-filled
+ * pages). This is used to make the test more reliable and avoid segfaulting
+ * when we return from the signal handler. Since we are making the signal
+ * handler's ucontext point to newly allocated memory, when that memory is
+ * paged-in it will contain the expected content.
+ */
+static char backing_mem[UF_MEM_SIZE];
+
+static size_t pagesize;
+
+/*
+ * Return a chunk of at least 'size' bytes of memory that will be handled by
+ * userfaultfd. If 'backing_data' is not NULL, its content will be save to
+ * 'backing_mem' and then copied into the faulting pages when the page fault
+ * is handled.
+ */
+void *get_uf_mem(size_t size, void *backing_data)
+{
+	void *ret;
+
+	if (uf_mem_offset + size > UF_MEM_SIZE) {
+		fprintf(stderr, "Requesting more uf_mem than expected!\n");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = &uf_mem[uf_mem_offset];
+
+	/* Save the data that will be copied into the faulting page */
+	if (backing_data != NULL)
+		memcpy(&backing_mem[uf_mem_offset], backing_data, size);
+
+	/* Reserve the requested amount of uf_mem */
+	uf_mem_offset += size;
+	/* Keep uf_mem_offset aligned to the page size (round up) */
+	uf_mem_offset = (uf_mem_offset + pagesize - 1) & ~(pagesize - 1);
+
+	return ret;
+}
+
+void *fault_handler_thread(void *arg)
+{
+	struct uffd_msg msg;	/* Data read from userfaultfd */
+	long uffd;		/* userfaultfd file descriptor */
+	struct uffdio_copy uffdio_copy;
+	struct pollfd pollfd;
+	ssize_t nread, offset;
+
+	uffd = (long) arg;
+
+	for (;;) {
+		pollfd.fd = uffd;
+		pollfd.events = POLLIN;
+		if (poll(&pollfd, 1, -1) == -1) {
+			perror("poll() failed");
+			exit(EXIT_FAILURE);
+		}
+
+		nread = read(uffd, &msg, sizeof(msg));
+		if (nread == 0) {
+			fprintf(stderr, "read(): EOF on userfaultfd\n");
+			exit(EXIT_FAILURE);
+		}
+
+		if (nread == -1) {
+			perror("read() failed");
+			exit(EXIT_FAILURE);
+		}
+
+		/* We expect only one kind of event */
+		if (msg.event != UFFD_EVENT_PAGEFAULT) {
+			fprintf(stderr, "Unexpected event on userfaultfd\n");
+			exit(EXIT_FAILURE);
+		}
+
+		/*
+		 * We need to handle page faults in units of pages(!).
+		 * So, round faulting address down to page boundary.
+		 */
+		uffdio_copy.dst = msg.arg.pagefault.address & ~(pagesize-1);
+
+		offset = (char *) uffdio_copy.dst - uf_mem;
+		uffdio_copy.src = (unsigned long) &backing_mem[offset];
+
+		uffdio_copy.len = pagesize;
+		uffdio_copy.mode = 0;
+		uffdio_copy.copy = 0;
+		if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1) {
+			perror("ioctl-UFFDIO_COPY failed");
+			exit(EXIT_FAILURE);
+		}
+	}
+}
+
+void setup_uf_mem(void)
+{
+	long uffd;		/* userfaultfd file descriptor */
+	pthread_t thr;
+	struct uffdio_api uffdio_api;
+	struct uffdio_register uffdio_register;
+	int ret;
+
+	pagesize = sysconf(_SC_PAGE_SIZE);
+
+	/* Create and enable userfaultfd object */
+	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+	if (uffd == -1) {
+		perror("userfaultfd() failed");
+		exit(EXIT_FAILURE);
+	}
+	uffdio_api.api = UFFD_API;
+	uffdio_api.features = 0;
+	if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1) {
+		perror("ioctl-UFFDIO_API failed");
+		exit(EXIT_FAILURE);
+	}
+
+	/*
+	 * Create a private anonymous mapping. The memory will be demand-zero
+	 * paged, that is, not yet allocated. When we actually touch the memory
+	 * the related page will be allocated via the userfaultfd mechanism.
+	 */
+	uf_mem = mmap(NULL, UF_MEM_SIZE, PROT_READ | PROT_WRITE,
+		      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (uf_mem == MAP_FAILED) {
+		perror("mmap() failed");
+		exit(EXIT_FAILURE);
+	}
+
+	/*
+	 * Register the memory range of the mapping we've just mapped to be
+	 * handled by the userfaultfd object. In 'mode' we request to track
+	 * missing pages (i.e. pages that have not yet been faulted-in).
+	 */
+	uffdio_register.range.start = (unsigned long) uf_mem;
+	uffdio_register.range.len = UF_MEM_SIZE;
+	uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
+	if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1) {
+		perror("ioctl-UFFDIO_REGISTER");
+		exit(EXIT_FAILURE);
+	}
+
+	/* Create a thread that will process the userfaultfd events */
+	ret = pthread_create(&thr, NULL, fault_handler_thread, (void *) uffd);
+	if (ret != 0) {
+		fprintf(stderr, "pthread_create(): Error. Returned %d\n", ret);
+		exit(EXIT_FAILURE);
+	}
+}
+
+/*
+ * Assumption: the signal was delivered while userspace was in transactional or
+ * suspended state, i.e. uc->uc_link != NULL.
+ */
+void signal_handler(int signo, siginfo_t *si, void *uc)
+{
+	ucontext_t *ucp = uc;
+
+	/* Skip 'trap' after returning, otherwise we get a SIGTRAP again */
+	ucp->uc_link->uc_mcontext.regs->nip += 4;
+
+	ucp->uc_mcontext.v_regs =
+		get_uf_mem(sizeof(elf_vrreg_t), ucp->uc_mcontext.v_regs);
+
+	ucp->uc_link->uc_mcontext.v_regs =
+		get_uf_mem(sizeof(elf_vrreg_t), ucp->uc_link->uc_mcontext.v_regs);
+
+	ucp->uc_link = get_uf_mem(sizeof(ucontext_t), ucp->uc_link);
+}
+
+bool have_userfaultfd(void)
+{
+	long rc;
+
+	errno = 0;
+	rc = syscall(__NR_userfaultfd, -1);
+
+	return rc == 0 || errno != ENOSYS;
+}
+
+int tm_signal_pagefault(void)
+{
+	struct sigaction sa;
+	stack_t ss;
+
+	SKIP_IF(!have_htm());
+	SKIP_IF(!have_userfaultfd());
+
+	setup_uf_mem();
+
+	/*
+	 * Set an alternative stack that will generate a page fault when the
+	 * signal is raised. The page fault will be treated via userfaultfd,
+	 * i.e. via fault_handler_thread.
+	 */
+	ss.ss_sp = get_uf_mem(SIGSTKSZ, NULL);
+	ss.ss_size = SIGSTKSZ;
+	ss.ss_flags = 0;
+	if (sigaltstack(&ss, NULL) == -1) {
+		perror("sigaltstack() failed");
+		exit(EXIT_FAILURE);
+	}
+
+	sa.sa_flags = SA_SIGINFO | SA_ONSTACK;
+	sa.sa_sigaction = signal_handler;
+	if (sigaction(SIGTRAP, &sa, NULL) == -1) {
+		perror("sigaction() failed");
+		exit(EXIT_FAILURE);
+	}
+
+	/* Trigger a SIGTRAP in transactional state */
+	asm __volatile__(
+			"tbegin.;"
+			"beq    1f;"
+			"trap;"
+			"1: ;"
+			: : : "memory");
+
+	/* Trigger a SIGTRAP in suspended state */
+	asm __volatile__(
+			"tbegin.;"
+			"beq    1f;"
+			"tsuspend.;"
+			"trap;"
+			"tresume.;"
+			"1: ;"
+			: : : "memory");
+
+	return EXIT_SUCCESS;
+}
+
+int main(int argc, char **argv)
+{
+	/*
+	 * Depending on kernel config, the TM Bad Thing might not result in a
+	 * crash, instead the kernel never returns control back to userspace, so
+	 * set a tight timeout. If the test passes it completes almost
+	 * immediately.
+	 */
+	test_harness_set_timeout(2);
+	return test_harness(tm_signal_pagefault, "tm_signal_pagefault");
+}