From c622fbdb8d37796c35ab1e08240a18b4222e5c9f Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Thu, 17 Sep 2015 01:19:54 +0200
Subject: [PATCH] copy: be more careful when trying to reflink

---
 src/basic/copy.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/basic/copy.c b/src/basic/copy.c
index b8cbe644d443a..019b2764b9509 100644
--- a/src/basic/copy.c
+++ b/src/basic/copy.c
@@ -37,7 +37,11 @@ int copy_bytes(int fdf, int fdt, uint64_t max_bytes, bool try_reflink) {
         assert(fdt >= 0);
 
         /* Try btrfs reflinks first. */
-        if (try_reflink && max_bytes == (uint64_t) -1) {
+        if (try_reflink &&
+            max_bytes == (uint64_t) -1 &&
+            lseek(fdf, 0, SEEK_CUR) == 0 &&
+            lseek(fdt, 0, SEEK_CUR) == 0) {
+
                 r = btrfs_reflink(fdf, fdt);
                 if (r >= 0)
                         return r;