block not-script put several NUL characters in physical-device-path xenstore entry

[marmarek]

How to file a helpful issue

Qubes OS release

R4.2

Brief summary

xenstore physical-device-path entry includes NUL bytes, it shouldn't.

Steps to reproduce

1. Start a qube
2. List xenstore entries xenstore-ls -fp |grep physical-device-path

Expected behavior

/local/domain/0/backend/vbd/55/51760/physical-device-path = "/dev/loop7"   (n0,r55)

Actual behavior

Some physical-device-path have NUL bytes (looks like all related to modules.img)

/local/domain/0/backend/vbd/55/51760/physical-device-path = "/dev/loop7\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"   (n0,r55)

My guess is this part:

https://github.com/QubesOS/qubes-linux-utils/blob/96698e24834d7e5d11ffff8648853ce53e721587/not-script/not-script.c#L489-L494

    if (major(dev) == LOOP_MAJOR) {
        if ((unsigned)snprintf(buf, sizeof buf, "/dev/loop%" PRIu32,
                               (unsigned)minor(dev)) >= sizeof buf)
            abort();
        physdev_path = buf;
    }

It updates physdev_path, but not path_len.