fix cloned_binary fallback corner case

[lifubang]

Because the /run folder in all my linux cloud server have noexec mount flag. So if cloned_binary fallback to temp file copy, there will be nsenter: could not ensure we are a cloned binary: Permission denied error.

I think we can check noexec mount flag first before we clone binary.

I think this is a corner case, so it is not important. It may just only effect kernel < 3.17 .

Signed-off-by: Lifubang [email protected]

[cyphar]

I really don't like how complicated this fallback code is getting -- not to mention how complicated such a trivial fix has become. If /run is often mounted noexec, then we should not try to use it -- rather than this complicated fallback.

I'm of a mixed mind about using $HOME -- there are lots of users that don't have a $HOME and it's a bit odd to use /root for this. Also, what if the host has a read-only / -- this is the reason why I wanted to use /run in the first place (because in order for runc to work it must be able to write to /run). We could make our own tmpfs mount (like I did in #2006) but that's just adding more complexity and won't help rootless users.

[lifubang]

If /run is often mounted noexec, then we should not try to use it -- rather than this complicated fallback.

Runc's default state dir is in /run, users may not know what's the meaning of nsenter: could not ensure we are a cloned binary: Permission denied error. And for rootless user, the XDG_RUNTIME_DIR env often is /run/user/0.