fix: support bpf_core_type_exists()

In the previous version, when no target is found in BTF, coreCalculateFixups()
will mark its relocation as poison and cause failure when verified by Linux verifier.

This issue is fixed by using right fixup for "checksForExistence" type of relocations.

Signed-off-by: Shang-Wen Wang (Sam Wang) <sam_s_wang@trendmicro.com>

internal/btf/core.go

@@ -315,10 +315,16 @@ func coreCalculateFixups(byteOrder binary.ByteOrder, local Type, targets []Type,
 
 	if bestFixups == nil {
 		// Nothing at all matched, probably because there are no suitable
-		// targets at all. Poison everything!
+		// targets at all.
+		//
+		// Poison everything except checksForExistence.
 		bestFixups = make([]COREFixup, len(relos))
 		for i, relo := range relos {
-			bestFixups[i] = COREFixup{kind: relo.kind, poison: true}
+			if relo.kind.checksForExistence() {
+				bestFixups[i] = COREFixup{kind: relo.kind, local: 1, target: 0}
+			} else {
+				bestFixups[i] = COREFixup{kind: relo.kind, poison: true}
+			}
 		}
 	}
 

internal/btf/testdata/relocs_read.c

@@ -26,6 +26,12 @@ struct bits {
 	u64 f : 16, g : 30;
 };
 
+struct nonexist {
+	int non_exist;
+};
+
+enum nonexist_enum { NON_EXIST = 1 };
+
 // Perform a read from a subprog to ensure CO-RE relocations
 // occurring there are tracked and executed in the final linked program.
 __attribute__((noinline)) int read_subprog() {
@@ -89,6 +95,15 @@ __attribute__((noinline)) int read_subprog() {
 	if (BPF_CORE_READ_BITFIELD(&bar, g) != 0x15443322)
 		return __LINE__;
 
+	if (bpf_core_type_exists(struct nonexist) != 0)
+		return __LINE__;
+
+	if (bpf_core_field_exists(((struct nonexist *)0)->non_exist) != 0)
+		return __LINE__;
+
+	if (bpf_core_enum_value_exists(enum nonexist_enum, NON_EXIST) != 0)
+		return __LINE__;
+
 	return 0;
 }