From 591b3b168d949c45d5b5994332a3007767845434 Mon Sep 17 00:00:00 2001
Message-Id: <591b3b168d949c45d5b5994332a3007767845434.1638277575.git.dcaratti@redhat.com>
From: Paolo Abeni <paolo.abeni@gmail.com>
Date: Thu, 14 Oct 2021 05:05:54 +0200
Subject: [PATCH] mptcpize: use explicit file copy instead of rename() (#161)

The mentioned syscall fails if the involved files belong to
different fs, which is pretty much expected in the relevant
scenario (tmp file, in tmpfs, and unit file usually under the
root partition)

Instead use sendfile() to explicitly copy all the contents. Note
that we need to close and re-open the unit file, as sendfile()
expect a O_WRITE fd as the target.
---
 src/mptcpize.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/src/mptcpize.c b/src/mptcpize.c
index cb79e09..b502d75 100644
--- a/src/mptcpize.c
+++ b/src/mptcpize.c
@@ -13,6 +13,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <sys/sendfile.h>
 
 #include <argp.h>
 #include <dlfcn.h>
@@ -163,10 +164,12 @@ static int unit_update(int argc, char *argv[], int enable)
 	char *unit, *line = NULL;
 	int append_env = enable;
 	char dst_path[PATH_MAX];
+	off_t bytes_copied = 0;
+	struct stat fileinfo;
+	int dst, unit_fd;
 	size_t len = 0;
 	ssize_t read;
 	FILE *src;
-	int dst;
 
 	if (argc < 1) {
 		fprintf(stderr, "missing unit argument\n");
@@ -210,11 +213,24 @@ static int unit_update(int argc, char *argv[], int enable)
 		error(1, errno, "can't read from %s", unit);
 	free(line);
 	fclose(src);
-	close(dst);
 
-	if (rename(dst_path, unit) < 0)
-		error(1, errno, "can't rename %s to %s", dst_path, unit);
+	// copy back the modified file into the original unit
+	// note: avoid using rename, as it fails across filesystems
+	if (fstat(dst, &fileinfo) < 0)
+		error(1, errno, "can't stat %s", dst_path);
+
+	// re-open the unit file for writing
+	// mkstemp already opened the temporary file for R/W so we don't need
+	// to touch that file descriptor.
+	unit_fd = open(unit, O_TRUNC | O_RDWR);
+	if (unit_fd < 0)
+		error(1, errno, "can't open %s for writing", unit);
 
+	while (bytes_copied < fileinfo.st_size)
+		if (sendfile(unit_fd, dst, &bytes_copied, fileinfo.st_size - bytes_copied) < 0)
+			error(1, errno, "can't copy from %s to %s", dst_path, unit);
+
+	close(dst);
 	if (system("systemctl daemon-reload") != 0)
 		error(1, errno, "can't reload unit, manual 'systemctl daemon-reload' is required");
 
-- 
2.31.1