misc: Add Openat2

2 files changed, 58 insertions, 0 deletions

diff --git a/misc/misc.go b/misc/misc.go
index 5526797..572651f 100644
--- a/misc/misc.go
+++ b/misc/misc.go
@@ -1,8 +1,30 @@
 // Package misc provides miscellaneous functions.
 package misc
 
+import (
+	"errors"
+	"strings"
+)
+
+// Copy_map the map src to dst without clearing existing items in dst.
 func Copy_map[K comparable, V any](dst map[K]V, src map[K]V) {
 	for k, v := range src {
 		dst[k] = v
 	}
 }
+
+// String_to_byte_ptr returns a pointer to the first byte of a string. It
+// ensures that the returned pointer is null-terminated.
+func String_to_byte_ptr(s string) (*byte, error) {
+	// If the string already contains a null then whoever attempts to
+	// interpret this as a null-terminated string won't be able to see the
+	// whole string. This is probably not expected by the caller.
+	if strings.IndexByte(s, 0) != -1 {
+		return nil, Err_null_byte
+	}
+	buf := make([]byte, len(s)+1) // Zeros them out...
+	copy(buf, s)                  // ... so the last byte would be null.
+	return &buf[0], nil
+}
+
+var Err_null_byte = errors.New("string contains null byte")
diff --git a/misc/openat2.go b/misc/openat2.go
new file mode 100644
index 0000000..ab9f32d
--- /dev/null
+++ b/misc/openat2.go
@@ -0,0 +1,36 @@
+package misc
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+const SYS_OPENAT2 = 437
+
+type Open_how_t struct {
+	Flags   uint64
+	Mode    uint64
+	Resolve uint64
+}
+
+const (
+	RESOLVE_BENEATH       = 0x8
+	RESOLVE_IN_ROOT       = 0x10
+	RESOLVE_NO_MAGICLINKS = 0x2
+	RESOLVE_NO_SYMLINKS   = 0x4
+	RESOLVE_NO_XDEV       = 0x1
+)
+
+// See openat2(2) on Linux
+func Openat2(dirfd int, path string, open_how *Open_how_t, size int) (fd int, err error) {
+	path_ptr, err := String_to_byte_ptr(path)
+	if err != nil {
+		return
+	}
+	_fd, _, errno := syscall.Syscall6(SYS_OPENAT2, uintptr(dirfd), uintptr(unsafe.Pointer(path_ptr)), uintptr(unsafe.Pointer(open_how)), uintptr(size), 0, 0)
+	fd = int(_fd)
+	if errno != 0 {
+		err = errno
+	}
+	return
+}