On Fri, Jun 23, 2017 at 12:01 AM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote: > Kees Cook <keescook@xxxxxxxxxxxx> writes: > >> Now that explicitly executed loaders are loaded in the mmap region, >> position PIE binaries lower in the address space to avoid possible >> collisions with mmap or stack regions. For 64-bit, align to 4GB to >> allow runtimes to use the entire 32-bit address space for 32-bit >> pointers. > > The change log and subject are a bit out of whack with the actual patch > because previously we used 512MB. > > How about? > > powerpc: Move ELF_ET_DYN_BASE to 4GB / 4MB > > Now that explicitly executed loaders are loaded in the mmap region, > we have more freedom to decide where we position PIE binaries in the > address space to avoid possible collisions with mmap or stack regions. > > For 64-bit, align to 4GB to allow runtimes to use the entire 32-bit > address space for 32-bit pointers. On 32-bit use 4MB. Good idea, thanks. I'll resend the series with the commit logs updated. > Is there any particular reasoning behind the 4MB value on 32-bit? So, I've dug around a bit on this, and I *think* the rationale is to avoid mapping a possible 4MB page table entry when it won't be using at least a portion near the lower end (NULL address area covered blocked by mmap_min_addr). It seems to be mainly tradition, though. > I gave this a quick spin and it booted OK on all my test boxes, which > covers 64-bit/32-bit kernel and userspace. So seems to work! Awesome, thanks for the testing! > Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > > cheers -Kees -- Kees Cook Pixel Security
