Re: [PATCH] New System call unshare (try 2)

From: Janak Desai
Date: Wed Oct 12 2005 - 13:18:09 EST

Jamie Lokier wrote:
Janak Desai wrote:

Don't allow namespace unsharing, if sharing fs (CLONE_FS)

Makes sense. clone() has the same test at the start. (I think
namespace should be a property of fs, not task, anyway. Or completely
eliminated because it's implied by the task's root dentry+vfsmnt).

Don't allow sighand unsharing if not unsharing vm

Why not? It's permitted to clone with unshared sighand and shared vm,
and it's useful too.

It's the combination shared sighand + unshared vm which is not
allowed by clone - so I think that's what you should refuse.

Yes, thanks. I had that backwards. I am checking for the illegal
combination of "shared sighand + unshared vm" later but I will
allow (and test) unsharing of sighand while keeping shared vm.

Don't allow vm unsharing if task cloned with CLONE_THREAD

It would be better to do what clone does, and say "don't allow sighand
unsharing if task cloned with CLONE_THREAD". This is because
CLONE_THREAD tasks must have shared signals.

In combination with the rule above for sighand (my rule, not yours),
that implies "don't allow vm unsharing.." as a consequence.

Ok. Basically, with my current logic you are forced to unshare
both vm and sighand. I will update so you can unshare just sighand
and tie the CLONE_THREAD restriction to sighand.

Don't allow vm unsharing if the task is performing async io

Why not?

Async ios are tied to an mm (see lookup_ioctx in fs/aio.c), which may
be shared among tasks. I see no reason why the async ios can't
continue and be waited in on in other tasks that may be using the old mm.

The new mm, if vm is unshared, would simply not see the outstanding
aios - in the same way as if a vm was unshared by fork().

Yes, I did see that async ios are tied to an mm and that aio context is
cleared when a new mm is created by clone/exec. It just appeared odd to
me that a task that setup aio is unable to continue performing aio
after unsharing mm, but the task that it was sharing mm with, can. I
don't have any problem making this change. It's just that I wasn't sure
about aio usage and thought it was better to not allow an mm unshare if
aio is being performed.


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