shm: Add mmap+memmove fallback if mremap() does not exist

Some operating systems (e.g. FreeBSD) do not implement mremap.
In that case we can grow the mapping by trying to map adjacent memory.
If that fails we can fall back to creating a new larger mapping and
moving the old memory contents there.

Co-authored-by: Koop Mast <kwm@rainbow-runner.nl>
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>

src/wayland-os.c

@@ -32,7 +32,9 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <string.h>
 #include <sys/epoll.h>
+#include <sys/mman.h>
 #include <sys/un.h>
 #ifdef HAVE_SYS_UCRED_H
 #include <sys/ucred.h>
@@ -210,3 +212,31 @@ wl_os_accept_cloexec(int sockfd, struct sockaddr *addr, socklen_t *addrlen)
 	fd = accept(sockfd, addr, addrlen);
 	return set_cloexec_or_close(fd);
 }
+
+/*
+ * Fallback function for operating systems that don't implement
+ * mremap(MREMAP_MAYMOVE).
+ */
+void *
+wl_os_mremap_maymove(int fd, void *old_data, ssize_t *old_size,
+		     ssize_t new_size, int prot, int flags)
+{
+	void *result;
+	/*
+	 * We could try mapping a new block immediately after the current one
+	 * with MAP_FIXED, however that is not guaranteed to work and breaks
+	 * on CHERI-enabled architectures since the data pointer will still
+	 * have the bounds of the previous allocation. As this is not a
+	 * performance-critical path, we always map a new region and copy the
+	 * old data to the new region.
+	 */
+	result = mmap(NULL, new_size, prot, flags, fd, 0);
+	if (result != MAP_FAILED) {
+		/* Copy the data over and unmap the old mapping. */
+		memcpy(result, old_data, *old_size);
+		if (munmap(old_data, *old_size) == 0) {
+			*old_size = 0; /* successfully unmapped old data. */
+		}
+	}
+	return result;
+}