nsenter: cloned_binary: userspace copy fallback if sendfile fails

There are some circumstances where sendfile(2) can fail (one example is
that AppArmor appears to block writing to deleted files with sendfile(2)
under some circumstances) and so we need to have a userspace fallback.
It's fairly trivial (and handles short-writes).

Signed-off-by: Aleksa Sarai <asarai@suse.de>

libcontainer/nsenter/cloned_binary.c

@@ -160,7 +160,7 @@ static char *read_file(char *path, size_t *length)
 
 	*length = 0;
 	for (;;) {
-		int n;
+		ssize_t n;
 
 		n = read(fd, buf, sizeof(buf));
 		if (n < 0)
@@ -403,6 +403,33 @@ static int try_bindfd(void)
 	return ret;
 }
 
+static ssize_t fd_to_fd(int outfd, int infd)
+{
+	ssize_t total = 0;
+	char buffer[4096];
+
+	for (;;) {
+		ssize_t nread, nwritten = 0;
+
+		nread = read(infd, buffer, sizeof(buffer));
+		if (nread < 0)
+			return -1;
+		if (!nread)
+			break;
+
+		do {
+			ssize_t n = write(outfd, buffer + nwritten, nread - nwritten);
+			if (n < 0)
+				return -1;
+			nwritten += n;
+		} while(nwritten < nread);
+
+		total += nwritten;
+	}
+
+	return total;
+}
+
 static int clone_binary(void)
 {
 	int binfd, execfd;
@@ -435,8 +462,12 @@ static int clone_binary(void)
 
 	while (sent < statbuf.st_size) {
 		int n = sendfile(execfd, binfd, NULL, statbuf.st_size - sent);
-		if (n < 0)
-			goto error_binfd;
+		if (n < 0) {
+			/* sendfile can fail so we fallback to a dumb user-space copy. */
+			n = fd_to_fd(execfd, binfd);
+			if (n < 0)
+				goto error_binfd;
+		}
 		sent += n;
 	}
 	close(binfd);