linker: fix fd leak in fixupKfuncs

The kfunc fixup code doesn't properly close the btf.Handle if the
compatibility check fails:

    leaked fd created at:
        github.com/cilium/ebpf/internal/sys.newFD+0x26
            /home/lorenz/dev/ebpf/internal/sys/fd.go:23
        github.com/cilium/ebpf/internal/sys.NewFD+0x24
            /home/lorenz/dev/ebpf/internal/sys/fd.go:63
        github.com/cilium/ebpf/internal/sys.BtfGetFdById+0x36
            /home/lorenz/dev/ebpf/internal/sys/types.go:583
        github.com/cilium/ebpf/btf.NewHandleFromID+0x2e
            /home/lorenz/dev/ebpf/btf/handle.go:83
        github.com/cilium/ebpf/btf.(*HandleIterator).Next+0xa4
            /home/lorenz/dev/ebpf/btf/handle.go:228
        github.com/cilium/ebpf.findTargetInModule+0xc9
            /home/lorenz/dev/ebpf/prog.go:965
        github.com/cilium/ebpf.findTargetInKernel+0xe4
            /home/lorenz/dev/ebpf/prog.go:943
        github.com/cilium/ebpf.fixupKfuncs+0x3ef
            /home/lorenz/dev/ebpf/linker.go:284
        github.com/cilium/ebpf.newProgramWithOptions+0x80d
            /home/lorenz/dev/ebpf/prog.go:277
        github.com/cilium/ebpf.(*collectionLoader).loadProgram+0x415
            /home/lorenz/dev/ebpf/collection.go:522
        github.com/cilium/ebpf.NewCollectionWithOptions+0x239
            /home/lorenz/dev/ebpf/collection.go:385
        github.com/cilium/ebpf.NewCollection+0x44
            /home/lorenz/dev/ebpf/collection.go:358
        github.com/cilium/ebpf.LoadCollection+0x45
            /home/lorenz/dev/ebpf/collection.go:680
        github.com/cilium/ebpf.TestInvalidKfunc+0xcb
            /home/lorenz/dev/ebpf/elf_reader_test.go:740
        testing.tRunner+0xfe
            /usr/local/go/src/testing/testing.go:1595
        runtime.goexit+0x0
            /usr/local/go/src/runtime/asm_amd64.s:1650

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

linker.go

@@ -40,10 +40,12 @@ func (hs handles) fdArray() []int32 {
 	return fda
 }
 
-func (hs handles) close() {
-	for _, h := range hs {
-		h.Close()
+func (hs *handles) Close() error {
+	var errs []error
+	for _, h := range *hs {
+		errs = append(errs, h.Close())
 	}
+	return errors.Join(errs...)
 }
 
 // splitSymbols splits insns into subsections delimited by Symbol Instructions.
@@ -231,7 +233,13 @@ func fixupAndValidate(insns asm.Instructions) error {
 // fixupKfuncs loops over all instructions in search for kfunc calls.
 // If at least one is found, the current kernels BTF and module BTFis are searched to set Instruction.Constant
 // and Instruction.Offset to the correct values.
-func fixupKfuncs(insns asm.Instructions) (handles, error) {
+func fixupKfuncs(insns asm.Instructions) (_ handles, err error) {
+	closeOnError := func(c io.Closer) {
+		if err != nil {
+			c.Close()
+		}
+	}
+
 	iter := insns.Iterate()
 	for iter.Next() {
 		ins := iter.Ins
@@ -250,6 +258,8 @@ fixups:
 	}
 
 	fdArray := make(handles, 0)
+	defer closeOnError(&fdArray)
+
 	for {
 		ins := iter.Ins
 
@@ -276,6 +286,11 @@ fixups:
 			return nil, err
 		}
 
+		idx, err := fdArray.add(module)
+		if err != nil {
+			return nil, err
+		}
+
 		if err := btf.CheckTypeCompatibility(kfm.Type, target.(*btf.Func).Type); err != nil {
 			return nil, &incompatibleKfuncError{kfm.Name, err}
 		}
@@ -285,11 +300,6 @@ fixups:
 			return nil, err
 		}
 
-		idx, err := fdArray.add(module)
-		if err != nil {
-			return nil, err
-		}
-
 		ins.Constant = int64(id)
 		ins.Offset = int16(idx)
 

prog.go

@@ -278,7 +278,7 @@ func newProgramWithOptions(spec *ProgramSpec, opts ProgramOptions) (*Program, er
 	if err != nil {
 		return nil, fmt.Errorf("fixing up kfuncs: %w", err)
 	}
-	defer handles.close()
+	defer handles.Close()
 
 	if len(handles) > 0 {
 		fdArray := handles.fdArray()
@@ -922,7 +922,12 @@ func findProgramTargetInKernel(name string, progType ProgramType, attachType Att
 	}
 
 	id, err := spec.TypeID(target)
-	return module, id, err
+	if err != nil {
+		module.Close()
+		return nil, 0, err
+	}
+
+	return module, id, nil
 }
 
 // findTargetInKernel attempts to find a named type in the current kernel.