HAMMER per-PFS permissions for ioctl syscalls

[Vasily Postnicov]

Hello. I have noticed, that some ioctls, like HAMMERIOC_GETHISTORY or
HAMMERIOC_GET_INFO can be made by any user, and there are some like
HAMMERIOC_ADD_SNAPSHOT, which only root can do. I find this somewhat
"unfair", because why a user cannot, for example, make a snapshot of his
own home directory, if there is a PFS mounted to that directory? I think
something like zfs allow/unallow is needed here. Any ideas how I can
implement this?

Maybe I should add a new record type to vfs/hammer/hammer_disk.h, say
HAMMER_RECTYPE_PERM, and use it in the similar way to
HAMMER_RECTYPE_CONFIG, like writing functions similar to
hammer_ioc_get/set_config? So when a user calls ioctl() it will be like
this in the kernel space:

1) Start a new transactions and initialize a cursor.
2) setup the cursor. Set cursor.key_beg.rec_type = HAMMER_RECTYPE_PERM;
3) do hammer_btree_lookup(&cursor);
4) If lookup succeeded, extract permission info and act accordingly to it.

So what you think? Will it work? Maybe I need to cache the results somehow
and do not call hammer_btree_lookup() each time ioctl is called? Or it is
already done automatically?
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