Hi

On Mon, Jun 2, 2014 at 12:59 PM, Hugh Dickins [off-list ref] wrote:

On Fri, 23 May 2014, David Herrmann wrote:

quoted

On Tue, May 20, 2014 at 4:20 AM, Hugh Dickins [off-list ref] wrote:

quoted

But this does highlight how the "size" arg to memfd_create() is
perhaps redundant.  Why give a size there, when size can be changed
afterwards?  I expect your answer is that many callers want to choose
the size at the beginning, and would prefer to avoid the extra call.
I'm not sure if that's a good enough reason for a redundant argument.

At one point in time we might be required to support atomic-sealing.
So a memfd_create() call takes the initial seals as upper 32bits in
"flags" and sets them before returning the object. If these seals
contain SEAL_GROW/SHRINK, we must pass the size during setup (think
CLOEXEC with fork()).

That does sound like over-design to me.  You stop short of passing
in an optional buffer of the data it's to contain, good.

I think it would be a clearer interface without the size, but really
that's an issue for the linux-api people you'll be Cc'ing next time.

You say "think CLOEXEC with fork()": you have thought about this, I
have not, please spell out for me what the atomic size guards against.
Do you want an fd that's not shared across fork?

My thinking was:
Imagine a seal called SEAL_OPEN that prevents against open()
(specifically on /proc/self/fd/). That seal obviously has to be set
before creating the object, otherwise there's a race. Therefore, I'd
need a "seals" argument for memfd_create(). Now imagine there's a
similar seal that has such a race but prevents any following resize.
Then I'd have to set the size during initialization, too.

However, in my opinion SEAL_OPEN does not protect against any real
attack (it only protects you from yourself). Therefore, I never added
it. Furthermore, I couldn't think of any similar situation, so I now
removed the "size" argument and made "flags" just an "unsigned int".
It was just a precaution, but I'm fine with dropping it as we cannot
come up with a real possible race.

Sorry for the confusion. I'll send v3 in a minute.

Thanks
David

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>