selftests/powerpc: Add test of fork with mapping above 512TB

This tests that when a process with a mapping above 512TB forks we
correctly separate the parent and child address spaces. This exercises
the bug in the context id handling fixed in the previous commit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/mm/.gitignore

@@ -3,4 +3,5 @@ subpage_prot
 tempfile
 prot_sao
 segv_errors
-wild_bctr
+wild_bctr
+large_vm_fork_separation

tools/testing/selftests/powerpc/mm/Makefile

@@ -2,7 +2,8 @@
 noarg:
 	$(MAKE) -C ../
 
-TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors wild_bctr
+TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors wild_bctr \
+		  large_vm_fork_separation
 TEST_GEN_FILES := tempfile
 
 top_srcdir = ../../../../..
@@ -13,6 +14,7 @@ $(TEST_GEN_PROGS): ../harness.c
 $(OUTPUT)/prot_sao: ../utils.c
 
 $(OUTPUT)/wild_bctr: CFLAGS += -m64
+$(OUTPUT)/large_vm_fork_separation: CFLAGS += -m64
 
 $(OUTPUT)/tempfile:
 	dd if=/dev/zero of=$@ bs=64k count=1

tools/testing/selftests/powerpc/mm/large_vm_fork_separation.c

@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0+
+//
+// Copyright 2019, Michael Ellerman, IBM Corp.
+//
+// Test that allocating memory beyond the memory limit and then forking is
+// handled correctly, ie. the child is able to access the mappings beyond the
+// memory limit and the child's writes are not visible to the parent.
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "utils.h"
+
+
+#ifndef MAP_FIXED_NOREPLACE
+#define MAP_FIXED_NOREPLACE	MAP_FIXED	// "Should be safe" above 512TB
+#endif
+
+
+static int test(void)
+{
+	int p2c[2], c2p[2], rc, status, c, *p;
+	unsigned long page_size;
+	pid_t pid;
+
+	page_size = sysconf(_SC_PAGESIZE);
+	SKIP_IF(page_size != 65536);
+
+	// Create a mapping at 512TB to allocate an extended_id
+	p = mmap((void *)(512ul << 40), page_size, PROT_READ | PROT_WRITE,
+		MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED_NOREPLACE, -1, 0);
+	if (p == MAP_FAILED) {
+		perror("mmap");
+		printf("Error: couldn't mmap(), confirm kernel has 4TB support?\n");
+		return 1;
+	}
+
+	printf("parent writing %p = 1\n", p);
+	*p = 1;
+
+	FAIL_IF(pipe(p2c) == -1 || pipe(c2p) == -1);
+
+	pid = fork();
+	if (pid == 0) {
+		FAIL_IF(read(p2c[0], &c, 1) != 1);
+
+		pid = getpid();
+		printf("child writing  %p = %d\n", p, pid);
+		*p = pid;
+
+		FAIL_IF(write(c2p[1], &c, 1) != 1);
+		FAIL_IF(read(p2c[0], &c, 1) != 1);
+		exit(0);
+	}
+
+	c = 0;
+	FAIL_IF(write(p2c[1], &c, 1) != 1);
+	FAIL_IF(read(c2p[0], &c, 1) != 1);
+
+	// Prevent compiler optimisation
+	barrier();
+
+	rc = 0;
+	printf("parent reading %p = %d\n", p, *p);
+	if (*p != 1) {
+		printf("Error: BUG! parent saw child's write! *p = %d\n", *p);
+		rc = 1;
+	}
+
+	FAIL_IF(write(p2c[1], &c, 1) != 1);
+	FAIL_IF(waitpid(pid, &status, 0) == -1);
+	FAIL_IF(!WIFEXITED(status) || WEXITSTATUS(status));
+
+	if (rc == 0)
+		printf("success: test completed OK\n");
+
+	return rc;
+}
+
+int main(void)
+{
+	return test_harness(test, "large_vm_fork_separation");
+}