Memfd_create() syscall merged into Linux tree

git.kernel.org

11 pointsjoshbaptiste4 comments

Blog post on memfd https://dvdhrm.wordpress.com/tag/memfd/

So previously there was a new syscall to replace /dev/urandom in cases where it can't be accessed, and now there's one to replace /dev/shm?

I know there are good practical reasons for both changes, but it seems like sandboxing is throwing the baby out with the bathwater a little bit. ;P

No: this API is fundamentally different from /dev/shm, and will hopefully obviate the need for some Android-specific patches (such as ashmem, though ashmem may already have been replaced by a new flavor of the week). /dev/shm is a folder, which represents a global namespace of memory objects, and which essentially must be manually reclaimed (imagine a world where you create an object, and then kill -9 the app before immediately uninstalling it). This API has a name, but it is for debugging purposes only: the lifetime and accessibility of the object is based purely on the Unix semantics of memory maps and file descriptors (which can be laterally shared using Unix domain sockets), allowing them to be automatically cleaned up. There is an additional advantage that it works without extra mount points, but I doubt that would have been a sufficient motivation for this change: /dev/shm is so problematic that it doesn't even exist on Android.

ashmem is still in use. This comes close to doing everything ashmem does, but is missing the pin/unpin mechanism, where a process can indicate pages that the system may reclaim if it needs the memory (and later attempt to pin them again to discover if they still exist or not).

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