writer: Use a union for stack buffer to ensure alignment

Adapted from an equivalent patch by Simon for ostree:
ostreedev/ostree@67ed2ac

Reported-by: Simon McVittie <[email protected]>
Signed-off-by: Colin Walters <[email protected]>

libcomposefs/lcfs-writer.c

@@ -568,13 +568,15 @@ int lcfs_compute_fsverity_from_fd(uint8_t *digest, int fd)
 // is an error if fsverity is not enabled.
 int lcfs_fd_measure_fsverity(uint8_t *digest, int fd)
 {
-	char buf[sizeof(struct fsverity_digest) + MAX_DIGEST_SIZE];
-	struct fsverity_digest *fsv = (struct fsverity_digest *)&buf;
+	union {
+		struct fsverity_digest fsv;
+		char buf[sizeof(struct fsverity_digest) + MAX_DIGEST_SIZE];
+	} result;
 
 	// First, ask the kernel if the file already has fsverity; if so we just return
 	// that.
-	fsv->digest_size = MAX_DIGEST_SIZE;
-	int res = ioctl(fd, FS_IOC_MEASURE_VERITY, fsv);
+	result.fsv.digest_size = MAX_DIGEST_SIZE;
+	int res = ioctl(fd, FS_IOC_MEASURE_VERITY, &result);
 	if (res == -1) {
 		if (errno == ENODATA || errno == EOPNOTSUPP || errno == ENOTTY) {
 			// Canonicalize errno
@@ -584,11 +586,11 @@ int lcfs_fd_measure_fsverity(uint8_t *digest, int fd)
 	}
 	// The file has fsverity enabled, but with an unexpected different algorithm (e.g. sha512).
 	// This is going to be a weird corner case.  For now, we error out.
-	if (fsv->digest_size != LCFS_DIGEST_SIZE) {
+	if (result.fsv.digest_size != LCFS_DIGEST_SIZE) {
 		return -EWRONGVERITY;
 	}
 
-	memcpy(digest, buf + sizeof(struct fsverity_digest), LCFS_DIGEST_SIZE);
+	memcpy(digest, result.buf + sizeof(struct fsverity_digest), LCFS_DIGEST_SIZE);
 
 	return 0;
 }