From e873af93371e5fc3796d2ec3f0ab2c43c026e039 Mon Sep 17 00:00:00 2001
From: Masayuki Morita <masayuki.morita@crowdworks.co.jp>
Date: Sat, 6 Jan 2018 00:40:48 +0900
Subject: [PATCH] Fix panic with substr interpolation function by invalid
 offset

Fixes #17041
---
 config/interpolate_funcs.go      | 2 +-
 config/interpolate_funcs_test.go | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/config/interpolate_funcs.go b/config/interpolate_funcs.go
index 58dee5cd9..72be817a0 100644
--- a/config/interpolate_funcs.go
+++ b/config/interpolate_funcs.go
@@ -1577,7 +1577,7 @@ func interpolationFuncSubstr() ast.Function {
 				return nil, fmt.Errorf("length should be a non-negative integer")
 			}
 
-			if offset > len(str) {
+			if offset > len(str) || offset < 0 {
 				return nil, fmt.Errorf("offset cannot be larger than the length of the string")
 			}
 
diff --git a/config/interpolate_funcs_test.go b/config/interpolate_funcs_test.go
index 9c993a89d..f63d81c30 100644
--- a/config/interpolate_funcs_test.go
+++ b/config/interpolate_funcs_test.go
@@ -2568,6 +2568,11 @@ func TestInterpolateFuncSubstr(t *testing.T) {
 				nil,
 				true,
 			},
+			{
+				`${substr("foo", -4, -1)}`,
+				nil,
+				true,
+			},
 
 			// invalid length
 			{