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Subjectsemantics of rhashtable and sysvipc

In sysvipc we have an ids->tables_initialized regarding the rhashtable,
introduced in 0cfb6aee70b (ipc: optimize semget/shmget/msgget for lots of keys).
It's there, specifically, to prevent nil pointer dereferences, from using an
uninitialized api. Considering how rhashtable_init() can fail (probably due to
ENOMEM, if anything), this made the overall ipc initialization capable of
failure as well. That alone is ugly, but fine, however I've spotted a few
issues regarding the semantics of tables_initialized (however unlikely they
may be):

- There is inconsistency in what we return to userspace: ipc_addid() returns
ENOSPC which is certainly _wrong_, while ipc_obtain_object_idr() returns

- After we started using rhashtables, ipc_findkey() can return nil upon
!tables_initialized, but the caller expects nil for when the ipc structure
isn't found, and can therefore call into ipcget() callbacks.

I see two possible fixes. The first is to return the proper error code
if !tables_initialized, however, I'm not sure how we want to deal with the
EINVAL cases when rhashtable_init() fails. Userspace has no reason to know
about this. The ENOMEM case I guess makes sense ok.

The second alternative would be to add a BUG_ON() if the initialization fails
and we get rid of all the tables_initialized hack. I know Linus isn't fond
of this, and in the past ipc has gotten rid of BUG_ON usage in ipc because
of this. However I mention it because there are other core areas that do this
(or call panic() straightaway). Ie in networking: sock_diag_init() and



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