pcapfilter: rename files and consolidate API

- Rename filter.go → bpf_filter.go, spec.go → bpf_spec.go
- Name stub layout constants in elfpatch_didi.go
- Consolidate function names (Apply/Load final form)
- Move README to docs/

Change-Id: I0c7806cdc02791f498fb609d1118f4f367186c51
(cherry picked from commit f4498302461c402688327619774c191677a9de5a)

Author: huhong-web

internal/pcapfilter/filter.go internal/pcapfilter/bpf_filter.gointernal/pcapfilter/filter.go renamed to internal/pcapfilter/bpf_filter.go

@@ -18,7 +18,7 @@
 // Usage:
 //
 //	spec, _ := ebpf.LoadCollectionSpec("prog.o")
-//	if err := pcapfilter.Apply(spec, "host 10.0.0.1"); err != nil { ... }
+//	if err := pcapfilter.InjectFilter(spec, "host 10.0.0.1"); err != nil { ... }
 //	coll, _ := ebpf.NewCollection(spec)
 //
 // Limitations inherited from go-pcap's pure-Go compiler:
@@ -60,6 +60,13 @@ const (
 	etherTypeIPv6 = 0x86dd
 )
 
+// L2StubSymbol and L3StubSymbol are the __noinline BPF function names that mark
+// the injection points. The C program must declare both pcap_stub_l2 and pcap_stub_l3.
+const (
+	L2StubSymbol = "pcap_stub_l2"
+	L3StubSymbol = "pcap_stub_l3"
+)
+
 // stack slot offsets used by patchPacketLoads.
 // R4/R5 hold packet data/data_end on entry; cbpfc scratch starts at +cbpfcStackOffset.
 // These slots must not overlap cbpfc's StackOffset region.

internal/pcapfilter/spec.go internal/pcapfilter/bpf_spec.gointernal/pcapfilter/spec.go renamed to internal/pcapfilter/bpf_spec.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !didi
+
 package pcapfilter
 
 import (
@@ -21,13 +23,6 @@ import (
 	"github.com/cilium/ebpf/asm"
 )
 
-// L2StubSymbol and L3StubSymbol are the __noinline BPF function names that mark
-// the injection points. The C program must declare both pcap_stub_l2 and pcap_stub_l3.
-const (
-	L2StubSymbol = "pcap_stub_l2"
-	L3StubSymbol = "pcap_stub_l3"
-)
-
 // Apply compiles filterExpr for both L2 (DLT_EN10MB) and L3 (DLT_RAW), then splices
 // the resulting eBPF programs into their matching stub functions in spec.
 // Must be called before ebpf.NewCollection.

internal/pcapfilter/load_default_test.go

@@ -62,7 +62,7 @@ func dumpPrograms(t *testing.T, spec *ebpf.CollectionSpec, prefix string) {
 	}
 }
 
-func TestApply(t *testing.T) {
+func TestInjectFilter(t *testing.T) {
 	if os.Getuid() != 0 {
 		t.Skip("Skipping: requires root")
 	}
@@ -79,7 +79,7 @@ func TestApply(t *testing.T) {
 
 	filterExpr := "ip and src net 192.168.1.0/24 and tcp dst port 3306"
 	if err := Apply(specs, filterExpr); err != nil {
-		t.Fatalf("Apply: %v", err)
+		t.Fatalf("InjectFilter: %v", err)
 	}
 
 	dumpPrograms(t, specs, "Program")