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plugin/autopath: Fixes a nil pointer dereference panic in autopath during search path walk (#8517)

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* plugin/autopath: Fixes a nil pointer dereference panic in autopath during search path walk

When a plugin later in the chain returns a ClientWrite rcode without
writing a response (for example acl's drop action, which returns
(dns.RcodeSuccess, nil) without calling WriteMsg), autopath dereferences
a nil nw.Msg at nw.Msg.Rcode and panics. The final fallback
w.WriteMsg(firstReply) can also receive a nil firstReply for the same
reason.

Skip search path elements that produced no message, and only write the
first reply when it is non-nil. This mirrors the nil guards recently
added in plugin/minimal (#8506), plugin/dns64 (#8511) and plugin/cache
(#8512).

Signed-off-by: zjncs <18910855655@163.com>

* plugin/autopath: silence unused-parameter lint and assert no client write

Address review feedback on #8517: rename the unused 'w' parameter in
TestAutoPathNilMsgFromNext to '_w' so the revive unused-parameter check
passes, and assert that the recorder receives no message so the intended
drop/no-client-write behavior is explicit.

Signed-off-by: zjncs <18910855655@163.com>

---------

Signed-off-by: zjncs <18910855655@163.com>
This commit is contained in:
Zhao Jianing
2026-09-05 17:20:27 +08:00
committed by GitHub
parent b6987aeb4a
commit b564fcd869
2 changed files with 28 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
plugin/autopath/autopath.go
View File

@@ -123,6 +123,10 @@ func (a *AutoPath) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
continue
}
if nw.Msg == nil {
continue
}
if nw.Msg.Rcode == dns.RcodeNameError {
continue
}
@@ -135,7 +139,7 @@ func (a *AutoPath) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
autoPathCount.WithLabelValues(metrics.WithServer(ctx)).Add(1)
return rcode, err
}
if plugin.ClientWrite(firstRcode) {
if plugin.ClientWrite(firstRcode) && firstReply != nil {
w.WriteMsg(firstReply)
}
return firstRcode, firstErr

23
plugin/autopath/autopath_test.go
View File

@@ -106,6 +106,29 @@ func TestAutoPathNoAnswer(t *testing.T) {
}
}
func TestAutoPathNilMsgFromNext(t *testing.T) {
ap := new(AutoPath)
ap.Zones = []string{"."}
// Simulate a plugin like acl's drop action that returns success
// without writing a response.
ap.Next = test.HandlerFunc(func(_ctx context.Context, _w dns.ResponseWriter, _r *dns.Msg) (int, error) {
return dns.RcodeSuccess, nil
})
ap.search = []string{"example.org.", "example.com.", "com.", ""}
m := new(dns.Msg)
m.SetQuestion("b.example.org.", dns.TypeA)
rec := dnstest.NewRecorder(&test.ResponseWriter{})
_, err := ap.ServeDNS(context.TODO(), rec, m)
if err != nil {
t.Fatalf("Expected no error, got %v", err)
}
if rec.Msg != nil {
t.Fatalf("Expected no client write, but the recorder got a message")
}
}
// nextHandler returns a Handler that returns an answer for the question in the
// request per the domain->answer map. On success an RR will be returned: "qname 3600 IN A 127.0.0.53"
func nextHandler(mm map[string]int) test.Handler {