elf: refuse initialized BTF maps

BTF map definitions work by encoding map type, key size, etc. in a struct.
Most of the time, the declared map is never initialized, even though space
is allocated for it in the ELF. It is possible to intialize the map:

    struct {
        __uint(type, BPF_MAP_TYPE_ARRAY_OF_MAPS);
        __uint(max_entries, 4);
        __type(key, int);
        __array(values, struct ringbuf_map);
    } ringbuf_arr SEC(".maps") = {
        .values = {
            [0] = &ringbuf1,
            [2] = &ringbuf2,
        },
    };

libbpf uses this for magic nested map initialization. Since we don't
support this at the moment, check that the contents of a BTF map are
zero, and bail out with internal.ErrNotSupported otherwise.

Author: lmb

elf_reader.go

@@ -444,6 +444,11 @@ func (ec *elfCode) loadBTFMaps(maps map[string]*MapSpec, mapSections map[elf.Sec
 	}
 
 	for idx, sec := range mapSections {
+		_, err := io.Copy(internal.DiscardZeroes{}, bufio.NewReader(sec.Open()))
+		if err != nil {
+			return fmt.Errorf("section %v: initializing BTF map definitions: %w", sec.Name, internal.ErrNotSupported)
+		}
+
 		syms := ec.symbolsPerSection[idx]
 		if len(syms) == 0 {
 			return fmt.Errorf("section %v: no symbols", sec.Name)

elf_reader_test.go

@@ -1,6 +1,7 @@
 package ebpf
 
 import (
+	"errors"
 	"flag"
 	"io/ioutil"
 	"os"
@@ -192,6 +193,16 @@ func TestLoadInvalidMapMissingSymbol(t *testing.T) {
 	})
 }
 
+func TestLoadInitializedBTFMap(t *testing.T) {
+	testutils.TestFiles(t, "testdata/initialized_btf_map-*.elf", func(t *testing.T, file string) {
+		_, err := LoadCollectionSpec(file)
+		t.Log(err)
+		if !errors.Is(err, internal.ErrNotSupported) {
+			t.Fatal("Loading an initialized BTF map should be unsupported")
+		}
+	})
+}
+
 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 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
+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 initialized_btf_map-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 initialized_btf_map-eb.elf
 
 clean:
 	-$(RM) *.elf

testdata/initialized_btf_map-eb.elf

@@ -0,0 +1,17 @@
+/* This file excercises the ELF loader. It is not a valid BPF program. */
+
+#include "common.h"
+
+#if __clang_major__ >= 9
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__type(key, uint32_t);
+	__type(value, uint64_t);
+	__uint(max_entries, 1);
+} hash_map __section(".maps") = {
+	/* This forces a non-zero byte into the .maps section. */
+	.key = (void *)1,
+};
+#else
+#error This file has to be compiled with clang >= 9
+#endif