[PATCH 2/3] t/helper/test-delta: segfault on OOB access

[Jann Horn <jannh@google.com>]

This ensures that any attempts to access memory directly after the input
buffer or delta buffer in a delta test will cause a segmentation fault.

Inspired by vsftpd.

Signed-off-by: Jann Horn <jannh@google.com>
---
 t/helper/test-delta.c | 78 +++++++++++++++++++++++++++++--------------
 1 file changed, 53 insertions(+), 25 deletions(-)

diff --git a/t/helper/test-delta.c b/t/helper/test-delta.c
index 34c725924..64d0ec902 100644
--- a/t/helper/test-delta.c
+++ b/t/helper/test-delta.c
@@ -16,45 +16,73 @@
 static const char usage_str[] =
 	"test-tool delta (-d|-p) <from_file> <data_file> <out_file>";
 
-int cmd__delta(int argc, const char **argv)
+/*
+ * We want to detect OOB reads behind the resulting buffer, even in non-ASAN
+ * builds. This helper reads some data into memory, aligns the *end* of the
+ * buffer on a page boundary, and reserves the next virtual page. This ensures
+ * that a single-byte OOB access segfaults.
+ */
+static void *map_with_adjacent_trailing_guard(const char *path,
+					      unsigned long *sizep)
 {
 	int fd;
 	struct stat st;
-	void *from_buf, *data_buf, *out_buf;
-	unsigned long from_size, data_size, out_size;
+	unsigned long page_size = getpagesize();
+	unsigned long padded_size, padding;
 
-	if (argc != 5 || (strcmp(argv[1], "-d") && strcmp(argv[1], "-p"))) {
-		fprintf(stderr, "usage: %s\n", usage_str);
-		return 1;
+	fd = open(path, O_RDONLY);
+	if (fd < 0) {
+		perror(path);
+		return NULL;
 	}
+	if (fstat(fd, &st)) {
+		perror(path);
+		close(fd);
+		return NULL;
+	}
+	*sizep = st.st_size;
 
-	fd = open(argv[2], O_RDONLY);
-	if (fd < 0 || fstat(fd, &st)) {
-		perror(argv[2]);
-		return 1;
+	/* pad in front for alignment and add trailing page */
+	padded_size = ((page_size-1) + st.st_size + page_size) & ~(page_size-1);
+	padding = padded_size - (st.st_size + page_size);
+
+	char *mapping = mmap(NULL, padded_size, PROT_READ|PROT_WRITE,
+			     MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+	if (mapping == MAP_FAILED ||
+	    mprotect(mapping + padded_size - page_size, page_size, PROT_NONE)) {
+		perror("mmap");
+		close(fd);
+		return NULL;
 	}
-	from_size = st.st_size;
-	from_buf = mmap(NULL, from_size, PROT_READ, MAP_PRIVATE, fd, 0);
-	if (from_buf == MAP_FAILED) {
-		perror(argv[2]);
+	if (read_in_full(fd, mapping + padding, st.st_size) != st.st_size) {
+		perror("read_in_full");
+		munmap(mapping, padded_size);
 		close(fd);
-		return 1;
+		return NULL;
 	}
+	mprotect(mapping, padded_size - page_size, PROT_READ);
 	close(fd);
+	return mapping + padding;
+}
 
-	fd = open(argv[3], O_RDONLY);
-	if (fd < 0 || fstat(fd, &st)) {
-		perror(argv[3]);
+int cmd__delta(int argc, const char **argv)
+{
+	int fd;
+	void *from_buf, *data_buf, *out_buf;
+	unsigned long from_size, data_size, out_size;
+
+	if (argc != 5 || (strcmp(argv[1], "-d") && strcmp(argv[1], "-p"))) {
+		fprintf(stderr, "usage: %s\n", usage_str);
 		return 1;
 	}
-	data_size = st.st_size;
-	data_buf = mmap(NULL, data_size, PROT_READ, MAP_PRIVATE, fd, 0);
-	if (data_buf == MAP_FAILED) {
-		perror(argv[3]);
-		close(fd);
+
+	from_buf = map_with_adjacent_trailing_guard(argv[2], &from_size);
+	if (from_buf == NULL)
+		return 1;
+
+	data_buf = map_with_adjacent_trailing_guard(argv[3], &data_size);
+	if (data_buf == NULL)
 		return 1;
-	}
-	close(fd);
 
 	if (argv[1][1] == 'd')
 		out_buf = diff_delta(from_buf, from_size,
-- 
2.19.0.rc0.228.g281dcd1b4d0-goog