elf: support weak functions

This commit adds support for the weak attribute on global functions.
This attribute can be useful to prevent the compiler from changing the
prototype of the function on some callsites.

This change was also tested in Cilium with a new global, weak function.
Without this change, loading would fail with the following error:

    offset 122800: relocating instruction: call: snat_v6_needs_masquerade:
    unsupported binding: STB_WEAK

Signed-off-by: Paul Chaignon <[email protected]>

Author: pchaigno

elf_reader.go

@@ -588,7 +588,7 @@ func (ec *elfCode) relocateInstruction(ins *asm.Instruction, rel elf.Symbol) err
 
 			switch typ {
 			case elf.STT_NOTYPE, elf.STT_FUNC:
-				if bind != elf.STB_GLOBAL {
+				if bind != elf.STB_GLOBAL && bind != elf.STB_WEAK {
 					return fmt.Errorf("call: %s: %w: %s", name, errUnsupportedBinding, bind)
 				}
 

elf_reader_test.go

@@ -207,6 +207,12 @@ func TestLoadCollectionSpec(t *testing.T) {
 				SectionName: "other",
 				License:     "MIT",
 			},
+			"global_fn4": {
+				Name:        "global_fn4",
+				Type:        UnspecifiedProgram,
+				SectionName: "other",
+				License:     "MIT",
+			},
 			"static_fn": {
 				Name:        "static_fn",
 				Type:        UnspecifiedProgram,
@@ -266,7 +272,7 @@ func TestLoadCollectionSpec(t *testing.T) {
 		}
 
 		ret := mustRun(t, coll.Programs["xdp_prog"], nil)
-		qt.Assert(t, qt.Equals(ret, 7))
+		qt.Assert(t, qt.Equals(ret, 11))
 	})
 }
 

info_test.go

@@ -523,12 +523,13 @@ func TestProgInfoExtBTF(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	expectedLineInfoCount := 28
+	expectedLineInfoCount := 33
 	expectedFuncInfo := map[string]bool{
 		"xdp_prog":   false,
 		"static_fn":  false,
 		"global_fn2": false,
 		"global_fn3": false,
+		"global_fn4": false,
 	}
 
 	lineInfoCount := 0

testdata/loader-clang-14-eb.elf

@@ -115,8 +115,12 @@ int __attribute__((noinline)) __section("other") global_fn3(uint32_t arg) {
 	return arg + 1;
 }
 
+int __attribute__((noinline)) __attribute__((weak)) __section("other") global_fn4(uint32_t arg) {
+	return arg + 3;
+}
+
 int __attribute__((noinline)) global_fn(uint32_t arg) {
-	return static_fn(arg) + global_fn2(arg) + global_fn3(arg);
+	return static_fn(arg) + global_fn2(arg) + global_fn3(arg) + global_fn4(arg);
 }
 
 volatile unsigned int key1       = 0; // .bss