fix: concat buffer bug (vyperlang#3738)

the `concat()` builtin did not respect the `copy_bytes()` API, it
allocated a buffer in some cases which did not have enough padding.

patches GHSA-2q8v-3gqq-4f8p

---------

Co-authored-by: cyberthirst <cyberthirst.eth@gmail.com>

tests/functional/builtins/codegen/test_concat.py

@@ -55,6 +55,70 @@ def krazykonkat(z: Bytes[10]) -> Bytes[25]:
     print("Passed third concat test")
 
 
+def test_concat_buffer(get_contract):
+    # GHSA-2q8v-3gqq-4f8p
+    code = """
+@internal
+def bar() -> uint256:
+    sss: String[2] = concat("a", "b")
+    return 1
+
+
+@external
+def foo() -> int256:
+    a: int256 = -1
+    b: uint256 = self.bar()
+    return a
+    """
+    c = get_contract(code)
+    assert c.foo() == -1
+
+
+def test_concat_buffer2(get_contract):
+    # GHSA-2q8v-3gqq-4f8p
+    code = """
+i: immutable(int256)
+
+@external
+def __init__():
+    i = -1
+    s: String[2] = concat("a", "b")
+
+@external
+def foo() -> int256:
+    return i
+    """
+    c = get_contract(code)
+    assert c.foo() == -1
+
+
+def test_concat_buffer3(get_contract):
+    # GHSA-2q8v-3gqq-4f8p
+    code = """
+s: String[1]
+s2: String[33]
+s3: String[34]
+
+@external
+def __init__():
+    self.s = "a"
+    self.s2 = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" # 33*'a'
+
+@internal
+def bar() -> uint256:
+    self.s3 = concat(self.s, self.s2)
+    return 1
+
+@external
+def foo() -> int256:
+    i: int256 = -1
+    b: uint256 = self.bar()
+    return i
+    """
+    c = get_contract(code)
+    assert c.foo() == -1
+
+
 def test_concat_bytes32(get_contract_with_gas_estimation):
     test_concat_bytes32 = """
 @external

vyper/builtins/functions.py

@@ -543,13 +543,12 @@ def build_IR(self, expr, context):
         else:
             ret_typ = BytesT(dst_maxlen)
 
+        # respect API of copy_bytes
+        bufsize = dst_maxlen + 32
+        buf = context.new_internal_variable(BytesT(bufsize))
+
         # Node representing the position of the output in memory
-        dst = IRnode.from_list(
-            context.new_internal_variable(ret_typ),
-            typ=ret_typ,
-            location=MEMORY,
-            annotation="concat destination",
-        )
+        dst = IRnode.from_list(buf, typ=ret_typ, location=MEMORY, annotation="concat destination")
 
         ret = ["seq"]
         # stack item representing our current offset in the dst buffer