On Fri, Jun 13, 2014 at 8:15 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote: > Hi > > On Fri, Jun 13, 2014 at 5:10 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> On Fri, Jun 13, 2014 at 3:36 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote: >>> Hi >>> >>> This is v3 of the File-Sealing and memfd_create() patches. You can find v1 with >>> a longer introduction at gmane: >>> http://thread.gmane.org/gmane.comp.video.dri.devel/102241 >>> An LWN article about memfd+sealing is available, too: >>> https://lwn.net/Articles/593918/ >>> v2 with some more discussions can be found here: >>> http://thread.gmane.org/gmane.linux.kernel.mm/115713 >>> >>> This series introduces two new APIs: >>> memfd_create(): Think of this syscall as malloc() but it returns a >>> file-descriptor instead of a pointer. That file-descriptor is >>> backed by anon-memory and can be memory-mapped for access. >>> sealing: The sealing API can be used to prevent a specific set of operations >>> on a file-descriptor. You 'seal' the file and give thus the >>> guarantee, that it cannot be modified in the specific ways. >>> >>> A short high-level introduction is also available here: >>> http://dvdhrm.wordpress.com/2014/06/10/memfd_create2/ >> >> Potentially silly question: is it guaranteed that mmapping and reading >> a SEAL_SHRINKed fd within size bounds will not SIGBUS? If so, should >> this be documented? (The particular issue here would be reading >> holes. It should work by using the zero page, but, if so, we should >> probably make it a real documented guarantee.) > > No, this is not guaranteed. See the previous discussion in v2 on Patch > 2/4 between Hugh and me. > > Summary is: If you want mmap-reads to not fail, use mlock(). There are > many situations where a fault might fail (think: OOM) and sealing is > not meant to protect against that. Btw., holes are automatically > filled with fresh pages by shmem. So a read only fails in OOM > situations (or memcg limits, etc.). > Isn't the point of SEAL_SHRINK to allow servers to mmap and read safely without worrying about SIGBUS? --Andy > Thanks > David -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>