bfd: fix partial write handling in bwritev()

When bfd is unbuffered, bwritev() used writev() directly, which can return
partial writes and leave checkpoint images incomplete.

This patch adds bfd_writev_all() helper that retries writev() until all
iovecs are written or an error occurs. This matches the pattern used by
write_all() for single writes.

Changes:
- Add bfd_writev_all() helper to retry writev() until completion
- Return 0 early when cnt == 0 to avoid zero-size edge cases
- Treat a zero-byte writev() result as EIO to avoid infinite loops
- Cast away const on the iov parameter for bfd_writev_all()

Signed-off-by: Ahmed Elaidy <elaidya225@gmail.com>

Author: 3idey

criu/bfd.c

@@ -275,16 +275,58 @@ int bwrite(struct bfd *bfd, const void *buf, int size)
 	return __bwrite(bfd, buf, size);
 }
 
+/*
+ * bfd_writev_all() behaves like writev() without the possibility of
+ * partial writes. Use only with blocking I/O.
+ */
+static ssize_t bfd_writev_all(int fd, struct iovec *iov, int cnt)
+{
+	ssize_t n = 0;
+
+	while (cnt > 0) {
+		ssize_t ret = writev(fd, iov, cnt);
+		if (ret == -1)
+			return ret;
+
+		if (!ret) {
+			errno = EIO;
+			return -1;
+		}
+
+		n += ret;
+
+		while (cnt > 0 && ret >= iov->iov_len) {
+			ret -= iov->iov_len;
+			iov++;
+			cnt--;
+		}
+
+		if (cnt > 0 && ret > 0) {
+			iov->iov_base = (char *)iov->iov_base + ret;
+			iov->iov_len -= ret;
+		}
+	}
+
+	return n;
+}
+
 int bwritev(struct bfd *bfd, const struct iovec *iov, int cnt)
 {
 	int i, written = 0;
 
 	if (!bfd_buffered(bfd)) {
-		/*
-		 * FIXME writev() should be called again if writev() writes
-		 * less bytes than requested.
-		 */
-		return writev(bfd->fd, iov, cnt);
+		ssize_t ret;
+
+		if (cnt <= 0) {
+			if (cnt < 0) {
+				errno = EINVAL;
+				return -1;
+			}
+			return 0;
+		}
+
+		ret = bfd_writev_all(bfd->fd, (struct iovec *)iov, cnt);
+		return ret;
 	}
 
 	for (i = 0; i < cnt; i++) {