From 344c3a7e95f9d8e20a1a7bd1a9480c6509b21146 Mon Sep 17 00:00:00 2001
From: Thomas Schoebel-Theuer <tst@1und1.de>
Date: Fri, 16 Apr 2021 11:23:17 +0200
Subject: [PATCH] copy: safeguard underflow

---
 kernel/mars_copy.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/mars_copy.c b/kernel/mars_copy.c
index 2a7ef6f9..3e20a3b0 100644
--- a/kernel/mars_copy.c
+++ b/kernel/mars_copy.c
@@ -750,7 +750,8 @@ int _run_copy(struct copy_brick *brick, loff_t this_start)
 
 	prev = -1;
 	if (this_start > brick->copy_last) {
-		prev = GET_INDEX(this_start - COPY_CHUNK);
+		if (this_start >= COPY_CHUNK)
+			prev = GET_INDEX(this_start - COPY_CHUNK);
 		max -= (this_start - brick->copy_last) / COPY_CHUNK;
 		all_max = max;
 	}