avoid calling munmap with zero length

mremap-fixed-huge-near-normal testcase mmaps 4*hpage_size - getpagesize()
of anon shared memory. Then it finds area of 3 huge pages aligned to
hpage_size and munmaps lower and upper areas:

   lower   +----- 3*hpage_size -----+   upper
 /       \/                          \/       \
 |--------|--------|--------|--------|--------|
 p        q                          |        p+xsize
                                     q+size

If initial mmap will place whole area in such way, that lower or upper
area size is zero, munmap fails with EINVAL and testcase fails.

mmap(2):
    EINVAL (since Linux 2.6.12) length was 0.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Eric Munson <emunson@bert.(none)>

1 files changed, 13 insertions, 7 deletions

diff --git a/tests/mremap-fixed-huge-near-normal.c b/tests/mremap-fixed-huge-near-normal.c
index 2c41813..e0f2ae4 100644
--- a/tests/mremap-fixed-huge-near-normal.c
+++ b/tests/mremap-fixed-huge-near-normal.c
@@ -87,6 +87,7 @@ void do_remap(int fd, void *target)
 void *map_align(size_t size, size_t align)
 {
 	unsigned long xsize = size + align - getpagesize();
+	size_t t;
 	void *p, *q;
 	int rc;
 
@@ -97,14 +98,19 @@ void *map_align(size_t size, size_t align)
 
 	q = PALIGN(p, align);
 
-	rc = munmap(p, q-p);
-	if (rc != 0)
-		FAIL("munmap(lower aligning): %s", strerror(errno));
-
-	rc = munmap(q + size, p + xsize - (q + size));
-	if (rc != 0)
-		FAIL("munmap(upper aligning): %s", strerror(errno));
+	t = q - p;
+	if (t) {
+		rc = munmap(p, t);
+		if (rc != 0)
+			FAIL("munmap(lower aligning): %s", strerror(errno));
+	}
 
+	t = p + xsize - (q + size);
+	if (t) {
+		rc = munmap(q + size, t);
+		if (rc != 0)
+			FAIL("munmap(upper aligning): %s", strerror(errno));
+	}
 
 	return q;
 }