libthr shared locks

Konstantin Belousov kostikbel at
Tue Feb 16 11:32:33 UTC 2016

On Mon, Feb 15, 2016 at 03:39:18PM -0600, Eric van Gyzen wrote:
> My only comment on kern_umtx.c is, why are the permission checks compiled out?
Assume that we changed the ABI of libthr and shared locks do not require
an offpage. Then, access to the locks is completely controlled by the
access to the page containing the lock. If a process can mmap the page,
it can own the lock.

>From this point of view, access to the offpage shared memory object
is the same as the access to the key page. Which is the reason to not
include the access permissions checks.

On the other hand, if you have something in kernel, which also owns a
reference on ucred (for other means), you sure consider whether the usual
access control is appropriate. Mostly, I leave the #ifdef-ed checks
to reconsider it later, which worked since you asked the question. I
definitely do not see much use of the shm_access() checks, but I am not
completely sure about possible mac policies utilization there, although
I do not really think they could be usefully attached to the app-level

> I also reviewed versions 2 and 3; the revisions look fine.
Thank you very much.

More information about the freebsd-threads mailing list