elf: detect map definitions with static qualifier

The following map definition

    static struct bpf_map_def foo __section("maps") = {...};

makes clang emit a relocation that points at the maps section, not
at the foo symbol (even though foo is still present in the symbol
table). The library currently doesn't recognize this and tries to
find a map named "maps" at load time, which is pretty confusing.

Try to detect the mistake by refusing direct map loads from the
"maps" and ".maps" (for BTF-style definitions) sections. They will
never succeed anyways.

elf_reader.go

@@ -291,10 +291,16 @@ outer:
 		// that goes via the helper. They are distinguished by
 		// different relocations.
 		switch typ {
+		// This is a direct load since the referenced symbol is a
+		// section. Weirdly, the offset of the real symbol in the
+		// section is encoded in the instruction stream.
 		case elf.STT_SECTION:
-			// This is a direct load since the referenced symbol is a
-			// section. Weirdly, the offset of the real symbol in the
-			// section is encoded in the instruction stream.
+			if name == "maps" || name == ".maps" {
+				// A direct load from the map definition sections doesn't make
+				// sense, this usually happens due to a stray `static`.
+				return fmt.Errorf("possible erroneous static qualifier on map definition: found reference to section %s", name)
+			}
+
 			if bind != elf.STB_LOCAL {
 				return fmt.Errorf("direct load: %s: unsupported relocation %s", name, bind)
 			}

elf_reader_test.go

@@ -182,6 +182,16 @@ func TestLoadInvalidMap(t *testing.T) {
 	})
 }
 
+func TestLoadInvalidMapMissingSymbol(t *testing.T) {
+	testutils.TestFiles(t, "testdata/invalid_map_static-el.elf", func(t *testing.T, file string) {
+		_, err := LoadCollectionSpec(file)
+		t.Log(err)
+		if err == nil {
+			t.Fatal("Loading a map with static qualifier should fail")
+		}
+	})
+}
+
 func TestLoadRawTracepoint(t *testing.T) {
 	testutils.SkipOnOldKernel(t, "4.17", "BPF_RAW_TRACEPOINT API")
 

testdata/Makefile

@@ -3,8 +3,8 @@ CLANG ?= $(LLVM_PREFIX)/clang
 CFLAGS := -target bpf -O2 -g -Wall -Werror $(CFLAGS)
 
 .PHONY: all clean
-all: loader-clang-6.0-el.elf loader-clang-7-el.elf loader-clang-8-el.elf loader-clang-9-el.elf invalid_map-el.elf raw_tracepoint-el.elf \
-	loader-clang-6.0-eb.elf loader-clang-7-eb.elf loader-clang-8-eb.elf loader-clang-9-eb.elf invalid_map-eb.elf raw_tracepoint-eb.elf
+all: loader-clang-6.0-el.elf loader-clang-7-el.elf loader-clang-8-el.elf loader-clang-9-el.elf invalid_map-el.elf raw_tracepoint-el.elf invalid_map_static-el.elf \
+	loader-clang-6.0-eb.elf loader-clang-7-eb.elf loader-clang-8-eb.elf loader-clang-9-eb.elf invalid_map-eb.elf raw_tracepoint-eb.elf invalid_map_static-eb.elf
 
 clean:
 	-$(RM) *.elf

testdata/invalid_map_static.c

@@ -0,0 +1,26 @@
+/* This file excercises the ELF loader. It is not a valid BPF program. */
+
+#include "common.h"
+
+struct bpf_map_def dummy __section("maps") = {
+	.type        = BPF_MAP_TYPE_HASH,
+	.key_size    = sizeof(uint32_t),
+	.value_size  = sizeof(uint64_t),
+	.max_entries = 1,
+	.map_flags   = 0,
+};
+
+/* The static qualifier leads to clang not emitting a symbol. */
+static struct bpf_map_def hash_map __section("maps") = {
+	.type        = BPF_MAP_TYPE_HASH,
+	.key_size    = sizeof(uint32_t),
+	.value_size  = sizeof(uint64_t),
+	.max_entries = 1,
+	.map_flags   = 0,
+};
+
+__section("xdp") int xdp_prog() {
+	uint32_t key = 0;
+	void *p      = map_lookup_elem(&hash_map, &key);
+	return !!p;
+}