There's a lot of repeated code in these functions. How much can be factored out into a new function that takes a block?

Locks aren't really intended for waiting for something to happen. Should use a condition variable. cond_t from util/sync.h is preferred because it allows the process to be interrupted by a signal, with e.g. ctrl-c.

I don't really like this way of getting the view controller. My other ideas are way more annoying to implement though.

I think a function `filesystem_register` that appends a filesystem to the array makes more sense than static filesystem ids.

Can you choose more meaningful magic values? 0x7265616c is used for realfs because it's "real" in ascii hex, so this means "ream". :)

Instead of a new function here, how about setting fd->ops after calling realfs_open?

Unused, please remove

At this point there should be a separate fs/real.h header.

Could you put these under Devices?

Seems to be a duplicate

Can this be moved into iosfs_mount? I think that would eliminate the need for a new do_mount_with_data.

Should include some information on what the error is

You're supposed to set mount->data after calling this, there shouldn't need to be a new function

startAccessingSecurityScopedResource doesn't set errno so this will return garbage

Can you create this statically to avoid having to call something to create it?