btf: Take instruction size into account when handling poison relocation

Makefile

@@ -47,6 +47,7 @@ TARGETS := \
 	btf/testdata/relocs \
 	btf/testdata/relocs_read \
 	btf/testdata/relocs_read_tgt \
+	btf/testdata/relocs_enum \
 	cmd/bpf2go/testdata/minimal
 
 .PHONY: all clean container-all container-shell generate

btf/core.go

@@ -43,7 +43,15 @@ func (f *COREFixup) Apply(ins *asm.Instruction) error {
 	if f.poison {
 		const badRelo = 0xbad2310
 
-		*ins = asm.BuiltinFunc(badRelo).Call()
+		// Relocation is poisoned, replace the instruction with an invalid one.
+
+		if ins.OpCode.IsDWordLoad() {
+			// Replace a dword load with a invalid dword load to preserve instruction size.
+			*ins = asm.LoadImm(asm.R10, badRelo, asm.DWord)
+		} else {
+			// Replace all single size instruction with a invalid call instruction.
+			*ins = asm.BuiltinFunc(badRelo).Call()
+		}
 		return nil
 	}
 

btf/core_reloc_test.go

@@ -110,3 +110,30 @@ func TestCORERelocationRead(t *testing.T) {
 		}
 	})
 }
+
+func TestLD64IMMReloc(t *testing.T) {
+	testutils.SkipOnOldKernel(t, "5.4", "vmlinux BTF in sysfs")
+
+	testutils.Files(t, testutils.Glob(t, "testdata/relocs_enum-*.elf"), func(t *testing.T, file string) {
+		fh, err := os.Open(file)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer fh.Close()
+
+		spec, err := ebpf.LoadCollectionSpecFromReader(fh)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if spec.ByteOrder != internal.NativeEndian {
+			return
+		}
+
+		coll, err := ebpf.NewCollection(spec)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer coll.Close()
+	})
+}

btf/testdata/relocs_enum.c

@@ -0,0 +1,16 @@
+#include "bpf_core_read.h"
+
+enum cgroup_subsys_id {
+	cpuset_cgrp_id,
+	cpuset_cgrp_id_lublub,
+	CGROUP_SUBSYS_COUNT,
+};
+
+#define __section(NAME) __attribute__((section(NAME), used))
+
+__section("socket/core_ld64imm") int core_ld64imm() {
+	if (bpf_core_enum_value_exists(enum cgroup_subsys_id, cpuset_cgrp_id_lublub)) {
+		__attribute__((unused)) const volatile int val = bpf_core_enum_value(enum cgroup_subsys_id, cpuset_cgrp_id_lublub);
+	}
+	return 0;
+}