memfd-bind: fixup systemd unit file and README

[kolyshkin]

The example of starting memfd-bind via systemd in README did not work
for me (Fedora 40, systemd 255):

# systemctl status memfd-bind@/usr/bin/runc
Invalid unit name "memfd-bind@/usr/bin/runc" escaped as "memfd-bind@-usr-bin-runc" (maybe you should use systemd-escape?).
   ○ memfd-bind@-usr-bin-runc.service
        Loaded: bad-setting (Reason: Unit memfd-bind@-usr-bin-runc.service has a bad unit file setting.)
        Active: inactive (dead)
          Docs: https://github.com/opencontainers/runc

So, let's use systemd-escape -p ("path") in the README example,
and use %f in the systemd unit file to prepend the slash to the
filename.

[rata]

LGTM

[kolyshkin]

@cyphar PTAL

[cyphar]

I can't seem to start this unit on my local machine:

% sudo systemctl start memfd-bind@-usr-bin-runc
Failed to start memfd-bind@-usr-bin-runc.service: Unit memfd-bind@-usr-bin-runc.service has a bad unit file setting.
See system logs and 'systemctl status memfd-bind@-usr-bin-runc.service' for details.
% journalctl -u memfd-bind@-usr-bin-runc
10月 14 21:01:27 thesius systemd[1]: /usr/local/lib/systemd/system/memfd-bind@.service:2: Failed to resolve unit specifiers in 'Manage memfd-bind of %f', ignoring: Invalid argument
10月 14 21:01:27 thesius systemd[1]: /usr/local/lib/systemd/system/memfd-bind@.service:7: Failed to resolve unit specifiers in '%f': Invalid argument
10月 14 21:01:27 thesius systemd[1]: memfd-bind@-usr-bin-runc.service: Unit configuration has fatal error, unit will not be started.

My systemd version should be new enough:

systemd 256 (256.6+suse.8.g8a0ae4d90a)
+PAM +AUDIT +SELINUX +APPARMOR +IMA -SMACK +SECCOMP +GCRYPT +GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +IPTC +KMOD +LIBCRYPTSETUP +LIBCRYPTSETUP_PLUGINS +LIBFDISK +PCRE2 +PWQUALITY +P11KIT +QRENCODE +TPM2 +BZIP2 +LZ4 +XZ +ZLIB +ZSTD +BPF_FRAMEWORK -XKBCOMMON -UTMP +SYSVINIT +LIBARCHIVE

Any idea what might be going wrong?

[kolyshkin]

% sudo systemctl start memfd-bind@-usr-bin-runc

The unit name should be memfd-bind@usr-bin-runc. This is how systemd-escape -p (and %f) work. Not sure why is this done this way, and the error message is not particularly helpful either.

[cyphar]

D'oh 🤦, yeah now it works. It would be nice for the "wrong" way of referencing it (memfd-bind@/usr/bin/runc) to continue to work but at least memfd-bind@usr/bin/runc still works.