elf_reader: support referencing weak variables

Our existing tests included a weak symbol, but it wasn't referenced in code and it ended up in .bss where other symbols sit at the same index. This confused the loader into thinking the symbol was referenced, though there were no actual symbol references to it in bpf code. Signed-off-by: Timo Beckers <[email protected]>
cilium · Oct 1, 2024 · b8fd6de · b8fd6de
1 parent a22a1cd
commit b8fd6de
Show file tree

Hide file tree

Showing 9 changed files with 16 additions and 6 deletions.
diff --git a/elf_reader.go b/elf_reader.go
@@ -519,7 +519,7 @@ func (ec *elfCode) relocateInstruction(ins *asm.Instruction, rel elf.Symbol) err
 
 		case elf.STT_OBJECT:
 			// LLVM 9 emits OBJECT-LOCAL symbols for anonymous constants.
-			if bind != elf.STB_GLOBAL && bind != elf.STB_LOCAL {
+			if bind != elf.STB_GLOBAL && bind != elf.STB_LOCAL && bind != elf.STB_WEAK {
 				return fmt.Errorf("direct load: %s: %w: %s", name, errUnsupportedBinding, bind)
 			}
 

diff --git a/elf_reader_test.go b/elf_reader_test.go
@@ -100,7 +100,7 @@ func TestLoadCollectionSpec(t *testing.T) {
 				Name:       SanitizeName(".bss", -1),
 				Type:       Array,
 				KeySize:    4,
-				ValueSize:  12,
+				ValueSize:  8,
 				MaxEntries: 1,
 			},
 			".data": {
@@ -117,6 +117,13 @@ func TestLoadCollectionSpec(t *testing.T) {
 				ValueSize:  4,
 				MaxEntries: 1,
 			},
+			".data.weak": {
+				Name:       SanitizeName(".data.weak", -1),
+				Type:       Array,
+				KeySize:    4,
+				ValueSize:  4,
+				MaxEntries: 1,
+			},
 			".data.struct": {
 				Name:       SanitizeName(".data.struct", -1),
 				Type:       Array,
@@ -209,7 +216,7 @@ func TestLoadCollectionSpec(t *testing.T) {
 			"neg":        {name: "neg", offset: 12, size: 4},
 			"struct_var": {name: "struct_var", offset: 0, size: 16},
 			"uneg":       {name: "uneg", offset: 8, size: 4},
-			"weak":       {name: "weak", offset: 8, size: 4},
+			"weak":       {name: "weak", offset: 0, size: 4},
 		},
 	}
 

diff --git a/testdata/loader-clang-11-eb.elf b/testdata/loader-clang-11-eb.elf
diff --git a/testdata/loader-clang-11-el.elf b/testdata/loader-clang-11-el.elf
diff --git a/testdata/loader-clang-14-eb.elf b/testdata/loader-clang-14-eb.elf
diff --git a/testdata/loader-clang-14-el.elf b/testdata/loader-clang-14-el.elf
diff --git a/testdata/loader-clang-17-eb.elf b/testdata/loader-clang-17-eb.elf
diff --git a/testdata/loader-clang-17-el.elf b/testdata/loader-clang-17-el.elf
diff --git a/testdata/loader.c b/testdata/loader.c
@@ -151,7 +151,9 @@ __hidden volatile uint32_t hidden;
 
 // Weak variables can be overridden by non-weak symbols when linking BPF
 // programs using bpftool. Make sure they appear in CollectionSpec.Variables.
-__weak volatile uint32_t weak;
+// Emit to its own section to avoid collisions with other symbols in .bss that
+// accidentally increase its refcount.
+__weak volatile uint32_t weak __section(".data.weak");
 
 struct struct_var_t {
 	uint64_t a;
@@ -161,8 +163,9 @@ volatile struct struct_var_t struct_var __section(".data.struct");
 
 __section("socket") int set_vars() {
 	hidden       = 0xbeef1;
-	struct_var.a = 0xbeef2;
-	struct_var.b = 0xbeef3;
+	weak         = 0xbeef2;
+	struct_var.a = 0xbeef3;
+	struct_var.b = 0xbeef4;
 	return 0;
 }