Hi On Fri, Jun 13, 2014 at 5:10 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Fri, Jun 13, 2014 at 3:36 AM, David Herrmann <dh.herrm...@gmail.com> 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.). Thanks David -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
