Hi

On Tue, Jun 17, 2014 at 11:54 AM, Florian Weimer [off-list ref] wrote:

On 06/13/2014 05:33 PM, David Herrmann wrote:

quoted

On Fri, Jun 13, 2014 at 5:17 PM, Andy Lutomirski [off-list ref]
wrote:

quoted

Isn't the point of SEAL_SHRINK to allow servers to mmap and read
safely without worrying about SIGBUS?

No, I don't think so.
The point of SEAL_SHRINK is to prevent a file from shrinking. SIGBUS
is an effect, not a cause. It's only a coincidence that "OOM during
reads" and "reading beyond file-boundaries" has the same effect:
SIGBUS.
We only protect against reading beyond file-boundaries due to
shrinking. Therefore, OOM-SIGBUS is unrelated to SEAL_SHRINK.

Anyone dealing with mmap() _has_ to use mlock() to protect against
OOM-SIGBUS. Making SEAL_SHRINK protect against OOM-SIGBUS would be
redundant, because you can achieve the same with SEAL_SHRINK+mlock().

I don't think this is what potential users expect because mlock requires
capabilities which are not available to them.

A couple of weeks ago, sealing was to be applied to anonymous shared memory.
Has this changed?  Why should *reading* it trigger OOM?

The file might have holes, therefore, you'd have to allocate backing
pages. This might hit a soft-limit and fail. To avoid this, use
fallocate() to allocate pages prior to mmap() or mlock() to make the
kernel lock them in memory.

Thanks
David