helper/seccomp: implement reader interface via pipe

This also does not require the libc tmpfile call.

BPF programs emitted by libseccomp seems to be deterministic. The tests would catch regressions as it verifies the program against known good output backed by manual testing.

Signed-off-by: Ophestra <cat@gensokyo.uk>

helper/seccomp/api.go

@@ -0,0 +1,51 @@
+package seccomp
+
+import (
+	"errors"
+	"io"
+	"os"
+	"syscall"
+)
+
+func Export(opts SyscallOpts) (f *os.File, err error) {
+	if f, err = tmpfile(); err != nil {
+		return
+	}
+	if err = exportFilter(f.Fd(), opts); err != nil {
+		return
+	}
+	_, err = f.Seek(0, io.SeekStart)
+	return
+}
+
+/*
+An Encoder writes a BPF program to an output stream.
+
+Methods of Encoder are not safe for concurrent use.
+
+An Encoder must not be copied after first use.
+*/
+type Encoder struct {
+	*exporter
+}
+
+func (e *Encoder) Read(p []byte) (n int, err error) {
+	if err = e.prepare(); err != nil {
+		return
+	}
+	return e.r.Read(p)
+}
+
+func (e *Encoder) Close() error {
+	if e.r == nil {
+		return syscall.EINVAL
+	}
+
+	// this hangs if the cgo thread fails to exit
+	return errors.Join(e.closeWrite(), <-e.exportErr)
+}
+
+// New returns an inactive Encoder instance.
+func New(opts SyscallOpts) *Encoder {
+	return &Encoder{newExporter(opts)}
+}