elf: refuse references to strings

Strings in BPF have to be stack allocated, so trying to reference
a string that is allocated in a dedicated section doesn't work. The
current error message is a bit confusing:

    load: .L.str: unsupported binding: STB_LOCAL

The most common case for this is a call to bpf_trace_printk:

    bpf_trace_printk("foo", sizeof("foo"));

Detect this case and return a better error.

Author: lmb

elf_reader.go

@@ -190,7 +190,7 @@ func (ec *elfCode) loadPrograms(progSections map[elf.SectionIndex]*elf.Section,
 
 		insns, length, err := ec.loadInstructions(sec, syms, relocations[idx])
 		if err != nil {
-			return nil, fmt.Errorf("program %s: can't unmarshal instructions: %w", funcSym.Name, err)
+			return nil, fmt.Errorf("program %s: %w", funcSym.Name, err)
 		}
 
 		progType, attachType, attachTo := getProgType(sec.Name)
@@ -271,15 +271,15 @@ func (ec *elfCode) relocateInstruction(ins *asm.Instruction, rel elf.Symbol) err
 		name = rel.Name
 	)
 
+	if int(rel.Section) > len(ec.Sections) {
+		return errors.New("out-of-bounds section index")
+	}
+
+	section := ec.Sections[int(rel.Section)]
 	if typ == elf.STT_SECTION {
 		// Symbols with section type do not have a name set. Get it
 		// from the section itself.
-		idx := int(rel.Section)
-		if idx > len(ec.Sections) {
-			return errors.New("out-of-bounds section index")
-		}
-
-		name = ec.Sections[idx].Name
+		name = section.Name
 	}
 
 outer:
@@ -323,6 +323,10 @@ outer:
 			fallthrough
 
 		case elf.STT_OBJECT:
+			if section.Flags&elf.SHF_STRINGS > 0 {
+				return fmt.Errorf("load: %s: string is not stack allocated: %w", name, ErrNotSupported)
+			}
+
 			if bind != elf.STB_GLOBAL {
 				return fmt.Errorf("load: %s: unsupported binding: %s", name, bind)
 			}

elf_reader_test.go

@@ -203,6 +203,16 @@ func TestLoadInitializedBTFMap(t *testing.T) {
 	})
 }
 
+func TestStringSection(t *testing.T) {
+	testutils.TestFiles(t, "testdata/strings-*.elf", func(t *testing.T, file string) {
+		_, err := LoadCollectionSpec(file)
+		t.Log(err)
+		if !errors.Is(err, ErrNotSupported) {
+			t.Error("References to a string section should be unsupported")
+		}
+	})
+}
+
 func TestLoadRawTracepoint(t *testing.T) {
 	testutils.SkipOnOldKernel(t, "4.17", "BPF_RAW_TRACEPOINT API")
 

testdata/Makefile

@@ -1,10 +1,18 @@
-LLVM_PREFIX ?= /usr/bin
-CLANG ?= $(LLVM_PREFIX)/clang
+CLANG ?= clang
 CFLAGS := -target bpf -O2 -g -Wall -Werror $(CFLAGS)
 
+TARGETS := loader-clang-6.0 \
+	loader-clang-7 \
+	loader-clang-8 \
+	loader-clang-9 \
+	invalid_map \
+	raw_tracepoint \
+	invalid_map_static \
+	initialized_btf_map \
+	strings
+
 .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 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
+all: $(addsuffix -el.elf,$(TARGETS)) $(addsuffix -eb.elf,$(TARGETS))
 
 clean:
 	-$(RM) *.elf

testdata/common.h

@@ -26,6 +26,8 @@ struct bpf_map_def {
 
 static void *(*map_lookup_elem)(const void *map, const void *key) = (void *)1;
 
+static long (*trace_printk)(const char *fmt, uint32_t fmt_size, ...) = (void *)6;
+
 static int (*perf_event_output)(const void *ctx, const void *map, uint64_t index, const void *data, uint64_t size) = (void *)25;
 
 static uint32_t (*get_smp_processor_id)(void) = (void *)8;

testdata/strings-eb.elf

@@ -0,0 +1,12 @@
+#include "common.h"
+
+char __license[] __section("license") = "MIT";
+
+#define STR1 "This string is allocated in the string section\n"
+#define STR2 "This one too\n"
+
+__section("socket") int filter() {
+	trace_printk(STR1, sizeof(STR1));
+	trace_printk(STR2, sizeof(STR2));
+	return 0;
+}