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SubjectRe: real POSIX.1b semaphores

From: Ulrich Drepper <>
Date: 20 Nov 1996 02:23:42 +0100

> Err.... be careful about the security issues. It should not be possible
> for a process which doesn't have access to a semaphore to just go into
> the shared page and much with the sempahores. This pretty much tosses
> out any idea where you use a single global page for all sempahores.....

Unfortunately there are not many security consideration in the POSIX
semaphores. Either they are process local or they are available
for any process. I read the section about semaphores twice and
found no sign for protection.

Gallmeister's book speaks about protection for named semaphores (and
it sounds reasonable). Nothing is specified for memory semaphores.
This really seems to be a whole in the standard.

I had read the earlier proposal to mean that named semaphores would also
go in globally accessible memory, which would have been a real problem.

Because of the security problems, I don't believe global unnamed
semaphores are useful. Process local semaphores are useful for places
where you need speed, and named semaphores for global access. But if
you have global unnamed semaphores with no security, they really are

- Ted

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