Allow null pointer as mount syscall options

[bjin]

I find that docker is not working properly with zfs 0.6.5 (See moby/moby#16258). The docker daemon log shows that mount failed with EINVAL for a newly created mountpoint=legacy dataset.

I write two simple program in C and Go, and it turns out that while mount("zroot/test", "/tmp/test", "zfs", 0, "") would succeed, mount("zroot/test", "/tmp/test", "zfs", 0, NULL) will trigger EINTVAL. The syscall Go package seems to prefer null pointer as mount options.

Additional tracing lead me to 0282c41, in zpl_parse_options(). What do you think of the idea to allow null mntopts here?

[behlendorf]

@bjin this was done intentionally to ensure certain hints were passed via mount options to the kernel via the mount helper. Assuming NULL was invalid was a convenient way to simplify the code. I didn't expect this to be an issue because I wasn't aware of any applications which bypassed mount.zfs. We can certainly allow a NULL here but I'm curious why docker is bypassing the helper. Without it certain zfs properties will be ignored and some other functionally won't work properly.

[aderouineau]

@behlendorf, The following 2 points in moby/moby/issues/16258 should be of interest.

@Mic92 mentions that:

zfs mount is not an option, because selinux labels have to passed with the mount.

@bjin referenced the following snippet from https://github.com/golang/sys/blob/master/unix/syscall_linux.go#L821

// Certain file systems get rather angry and EINVAL if you give
// them an empty string of data, rather than NULL.

[Mic92]

mount.zfs could be an option, since it supports selinux.
I will try this one. Do you know, if this is also part of other zfs distribution such as freebsd?

[behlendorf]

The bug here has been addressed in 0.6.5.1. But just so you're aware you can pass selinux contexts as a mount option using -o with ZoL. See mount.zfs(8). Alternately you can set this as dataset properties on Linux if that's more convenient.

zfs mount -o fscontext|defcontext|rootcontext tank/fs