On Tue, Apr 28, 2026 at 09:51:09PM +0200, David Hildenbrand (Arm) wrote: > On 4/27/26 17:34, Ard Biesheuvel wrote: > > From: Ard Biesheuvel <[email protected]> > > > > The empty zero page is used to back any kernel or user space mapping > > that is supposed to remain cleared, and so the page itself is never > > supposed to be modified. > > > > So make it __ro_after_init rather than __page_aligned_bss: on most > > architectures, this ensures that both the kernel's mapping of it and any > > aliases that are accessible via the kernel direct (linear) map are > > mapped read-only, and cannot be used (inadvertently or maliciously) to > > corrupt the contents of the zero page. > > > > Signed-off-by: Ard Biesheuvel <[email protected]> > > --- > > mm/mm_init.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/mm_init.c b/mm/mm_init.c > > index f9f8e1af921c..6ca01ed2a5a4 100644 > > --- a/mm/mm_init.c > > +++ b/mm/mm_init.c > > @@ -57,7 +57,7 @@ unsigned long zero_page_pfn __ro_after_init; > > EXPORT_SYMBOL(zero_page_pfn); > > > > #ifndef __HAVE_COLOR_ZERO_PAGE > > -uint8_t empty_zero_page[PAGE_SIZE] __page_aligned_bss; > > +uint8_t empty_zero_page[PAGE_SIZE] __ro_after_init __aligned(PAGE_SIZE); > > EXPORT_SYMBOL(empty_zero_page); > > > > struct page *__zero_page __ro_after_init; > > I am no expert on BSS etc, but from what I understand, we'll still get zeroed > page-aligned memory. I don't know if there is any other impact on not having > it > in bss.page_aligned. I assume no
IIUC, unlike BSS, it will be part of the kernel image. So kernel image size will grow by PAGE_SIZE. But compressor will eat it as it is all zeros, so it should be okay. -- Kiryl Shutsemau / Kirill A. Shutemov
