Re: RFC: named anonymous vmas

[Alex Elsayed <eternaleye@gmail.com>]

Colin Cross wrote:

> On Sat, Jun 22, 2013 at 3:31 AM, Christoph Hellwig <hch@infradead.org>
> wrote:
>> On Fri, Jun 21, 2013 at 04:42:41PM -0700, Colin Cross wrote:
>>> ranges, which John Stultz has been implementing.  The second is
>>> anonymous shareable memory without having a world-writable tmpfs that
>>> untrusted apps could fill with files.
>>
>> I still haven't seen any explanation of what ashmem buys over a shared
>> mmap of /dev/zero in that respect, btw.
> 
> I believe the difference is that ashmem ties the memory to an fd, so
> it can be passed to another process and mmaped to get to the same
> memory, but /dev/zero does not.  Passing a /dev/zero fd and mmaping it
> would result in a brand new region of zeroed memory.  Opening a tmpfs
> file would allow sharing memory by passing the fd, but we don't want a
> world-writable tmpfs.

Couldn't this be done by having a root-only tmpfs, and having a userspace 
component that creates per-app directories with restrictive permissions on 
startup/app install? Then each app creates files in its own directory, and 
can pass the fds around.

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