add mount options to sysfs

Solar,

While I've implemented gid= and umask= for sysfs as mount options, and for debugfs & configfs as sysctl variables (as they are not per-namespace, but global to the kernel), I think it makes much sense to implement CONFIG_{SYSFS,DEBUG_FS,CONFIGFS_FS}_{UMASK,GID} to be able to control permissions of files already created before userspace is able to change default permissions. Without configurable default umask/gid via CONFIG_* one should use "chmod -R /sys/" to alter already created files' perms, which is a bit inconsistent.

Actual restrictions are simple, but sysctl handler for octal values (umask) and support of octal CONFIG_* values are needed. The former already exists, but witout octal printing. You may do:

echo '0007' > /proc/sys/debug/debugfs_umask

and it is parsed correctly. But output will be decimal.

For CONFIG_*_UMASK one may not define octal values (more than 7), and numerics beginning from "0" are not parsed at all.

sysctl part is not hard, but for CONFIG_ parsing I have to dive into kbuild sources, which I didn't touch yet at all. Looking at the number of places where e.g. S_HEX is used (parsing of hex values) I suppose it will be time consuming task.

So, I think sysfs/debugfs/configfs umask=/gid= may be defered until all obviously necessary hardening features are applied into the upstream or at least discussed.

Thanks,

Vasiliy