On Tue, Jan 23, 2024 at 05:04:22PM -0800, Yang Shi wrote: > On Tue, Jan 23, 2024 at 2:37 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > On Tue, Jan 16, 2024 at 09:09:45AM +0100, Ard Biesheuvel wrote: > > > (cc Kees, LAKML) > > > > > > https://lkml.kernel.org/r/69fa6015256613ed10aee996e181ebd4%40horotw.com > > > > > > On Mon, 15 Jan 2024 at 21:46, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > > ... > > > > Yeah, I don't know either. Outside my scope of expertise. > > > > > > > > I received a suggestion off-list that we only do the PMD alignment on > > > > 64-bit, which seems quite reasonable to me. After all, I don't care > > > > about performance on 32-bit just as much as I don't care about security > > > > on 32-bit. > > > > > > > > > > For context, the culprit is > > > > > > commit 1854bc6e2420472676c5c90d3d6b15f6cd640e40 > > > Author: William Kucharski <william.kucharski@xxxxxxxxxx> > > > Date: Sun Sep 22 08:43:15 2019 -0400 > > > > > > mm/readahead: Align file mappings for non-DAX > > > > > > When we have the opportunity to use PMDs to map a file, we want to follow > > > the same rules as DAX. > > > > > > Signed-off-by: William Kucharski <william.kucharski@xxxxxxxxxx> > > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > > > > > which affects *all* 32-bit architectures not just i686. 32-bit ARM > > > user space is still being deployed widely, even on arm64 Chromebooks > > > running 64-bit kernels (at least up until recently) so unfortunately, > > > we're not quite at the point yet where we can just let it rot. > > > > Is this related at all to this thread as well? > > https://lore.kernel.org/lkml/20220809142457.4751229f@xxxxxxxxxxxxxxxxxxxx/ > > Yes > > > > > Can we avoid this on 32-bit or at least not mislead userspace about the > > available entropy visible in /proc/sys/vm/mmap_rnd*_bits ? > > https://lore.kernel.org/linux-mm/20240118133504.2910955-1-shy828301@xxxxxxxxx/ > > This patch basically made thp_get_unmapped_area no-op on 32 bit. Ah-ha! Okay, thanks very much. I missed this landing. :) -- Kees Cook
