system: Use symlink xattr functions from x/sys/unix

Use the symlink xattr syscall wrappers Lgetxattr and Lsetxattr from
x/sys/unix (introduced in golang/sys@b90f89a) instead of providing own
wrappers. Leave the functionality of system.Lgetxattr intact with
respect to the retry with a larger buffer, but switch it to use
unix.Lgetxattr. Also leave system.Lsetxattr intact (even though it's
just a wrapper around the corresponding function from unix) in order to
keep moby building for !linux.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

pkg/system/xattrs_linux.go

@@ -1,64 +1,29 @@
 package system
 
-import (
-	"unsafe"
-
-	"golang.org/x/sys/unix"
-)
+import "golang.org/x/sys/unix"
 
 // Lgetxattr retrieves the value of the extended attribute identified by attr
 // and associated with the given path in the file system.
 // It will returns a nil slice and nil error if the xattr is not set.
 func Lgetxattr(path string, attr string) ([]byte, error) {
-	pathBytes, err := unix.BytePtrFromString(path)
-	if err != nil {
-		return nil, err
-	}
-	attrBytes, err := unix.BytePtrFromString(attr)
-	if err != nil {
-		return nil, err
-	}
-
 	dest := make([]byte, 128)
-	destBytes := unsafe.Pointer(&dest[0])
-	sz, _, errno := unix.Syscall6(unix.SYS_LGETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(destBytes), uintptr(len(dest)), 0, 0)
+	sz, errno := unix.Lgetxattr(path, attr, dest)
 	if errno == unix.ENODATA {
 		return nil, nil
 	}
 	if errno == unix.ERANGE {
 		dest = make([]byte, sz)
-		destBytes := unsafe.Pointer(&dest[0])
-		sz, _, errno = unix.Syscall6(unix.SYS_LGETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(destBytes), uintptr(len(dest)), 0, 0)
+		sz, errno = unix.Lgetxattr(path, attr, dest)
 	}
-	if errno != 0 {
+	if errno != nil {
 		return nil, errno
 	}
 
 	return dest[:sz], nil
 }
 
-var _zero uintptr
-
 // Lsetxattr sets the value of the extended attribute identified by attr
 // and associated with the given path in the file system.
 func Lsetxattr(path string, attr string, data []byte, flags int) error {
-	pathBytes, err := unix.BytePtrFromString(path)
-	if err != nil {
-		return err
-	}
-	attrBytes, err := unix.BytePtrFromString(attr)
-	if err != nil {
-		return err
-	}
-	var dataBytes unsafe.Pointer
-	if len(data) > 0 {
-		dataBytes = unsafe.Pointer(&data[0])
-	} else {
-		dataBytes = unsafe.Pointer(&_zero)
-	}
-	_, _, errno := unix.Syscall6(unix.SYS_LSETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(dataBytes), uintptr(len(data)), uintptr(flags), 0)
-	if errno != 0 {
-		return errno
-	}
-	return nil
+	return unix.Lsetxattr(path, attr, data, flags)
 }