From 3d0fcc948d0dd7869b48024678d23e5d127dd211 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Janiszewski?= <janisozaur@gmail.com>
Date: Tue, 8 Dec 2015 23:53:43 +0100
Subject: [PATCH] POSIX: use mincore to detect impending doom

Call to mmap (or next malloc) will likely fail if there are already
mapped pages within the region we want to map ourselves. Using mincore
we can detect if such pages exist and warn user about what's going to
happen.
---
 src/openrct2.c | 39 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/src/openrct2.c b/src/openrct2.c
index 6445a55572..6bb2fcd81b 100644
--- a/src/openrct2.c
+++ b/src/openrct2.c
@@ -513,6 +513,43 @@ static bool openrct2_setup_rct2_segment()
 	off_t file_size = 6750208;
 
 	int len = 0x01429000 - 0x8a4000; // 0xB85000, 12079104 bytes or around 11.5MB
+	int pageSize = getpagesize();
+	int numPages = (len + pageSize - 1) / pageSize;
+	unsigned char *dummy = malloc(numPages);
+	int err = mincore((void *)0x8a4000, len, dummy);
+	bool pagesDirty = false;
+	if (err != 0)
+	{
+		err = errno;
+#ifdef __linux__
+		// On Linux ENOMEM means all requested range is unmapped
+		if (err != ENOMEM)
+		{
+			pagesDirty = true;
+			perror("mincore");
+		}
+#else
+		pagesDirty = true;
+		perror("mincore");
+#endif // __linux__
+	} else {
+		log_warning("mincore ok");
+		for (int i = 0; i < numPages; i++)
+		{
+			if (dummy[i] != 0)
+			{
+				pagesDirty = true;
+				void *start = (void *)0x8a4000 + i * pageSize;
+				void *end = (void *)0x8a4000 + (i + 1) * pageSize - 1;
+				log_warning("page %p - %p has flags: %x, you're in for bad time!", start, end, dummy[i]);
+			}
+		}
+	}
+	free(dummy);
+	if (pagesDirty)
+	{
+		log_error("Found already mapped pages in region we want to claim. This means something accessed memory before we got to and following mmap (or next malloc) call will likely fail.");
+	}
 	// section: rw data
 	gDataSegment = mmap((void *)0x8a4000, len, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_ANONYMOUS | MAP_SHARED, 0, 0);
 	if (gDataSegment != (void *)0x8a4000) {
@@ -530,7 +567,7 @@ static bool openrct2_setup_rct2_segment()
 	}
 
 	void *fbase = mmap(NULL, file_size, PROT_READ, MAP_PRIVATE, gExeFd, 0);
-	int err = errno;
+	err = errno;
 	log_warning("mmapped file to %p", fbase);
 	if (fbase == MAP_FAILED)
 	{