Re: [PATCH 3/4] elf: align ET_DYN base to exec folio order for contpte mapping

[hev]

On Sat, Mar 14, 2026 at 3:47 AM Usama Arif <usama.arif@xxxxxxxxx> wrote:
>
>
>
> On 13/03/2026 17:42, WANG Rui wrote:
> > Hi Usama,
> >
>
> Hello!
>
> > Glad to see you're pushing on this, I'm also following it. I first noticed this when rustc's perf regressed after a binutils upgrade. I'm trying to make ld.so to aware THP and adjust PT_LOAD alignment to increase the chances of shared libraries being mapped by THP [1]. As you're probably seen, I'm doing something similar in the kernel to improve it for executables [2].
>
> For us it came about because we use 64K page size on ARM, and none of the
> text sections were getting hugified (because PMD size is 512M). I went with
> exec_folio_order() = cont-pte size (2M) for 16K and 64K as we can get both page
> fault benefit (which might not be that important) and iTLB coverage (due to
> cont-pte).
> x86 already faults in at 2M (HPAGE_PMD_ORDER) due to force_thp_readahead path in
> do_sync_mmap_readahead() so the memory pressure introduced in ARM won't be worse
> than what already exists in x86.
>
> >
> >> +                    if (exec_folio_order())
> >> +                            alignment = max(alignment,
> >> +                                    (unsigned long)PAGE_SIZE << exec_folio_order());
> >
> > I’m curious, does it make sense to add some constraints here, like only increasing p_align when the segment length, virtual address, and file offset are all huge-aligned, as I did in my patch? This has come up several times in the glibc review, where increasing alignment was noted to reduce ASLR entropy.
> >
>
> Yes I think this makes sense!
>
> Although maybe we should check all segments with PT_LOAD. So maybe something
> like below over this series?
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 2d2b3e9fd474f..a0e83b541a7d8 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1116,10 +1116,30 @@ static int load_elf_binary(struct linux_binprm *bprm)
>                          * the hardware cannot coalesce PTEs (e.g. arm64
>                          * contpte) even though the physical memory and
>                          * file offset are correctly aligned.
> +                        *
> +                        * Only increase alignment when at least one
> +                        * PT_LOAD segment is large enough to contain a
> +                        * full folio and has its file offset and virtual
> +                        * address folio-aligned. This avoids reducing
> +                        * ASLR entropy for small binaries that cannot
> +                        * benefit from contpte mapping.
>                          */
> -                       if (exec_folio_order())
> -                               alignment = max(alignment,
> -                                       (unsigned long)PAGE_SIZE << exec_folio_order());
> +                       if (exec_folio_order()) {
> +                               unsigned long folio_sz = PAGE_SIZE << exec_folio_order();
> +
> +                               for (i = 0; i < elf_ex->e_phnum; i++) {
> +                                       if (elf_phdata[i].p_type != PT_LOAD)
> +                                               continue;
> +                                       if (elf_phdata[i].p_filesz < folio_sz)
> +                                               continue;
> +                                       if (!IS_ALIGNED(elf_phdata[i].p_vaddr, folio_sz))
> +                                               continue;
> +                                       if (!IS_ALIGNED(elf_phdata[i].p_offset, folio_sz))
> +                                               continue;
> +                                       alignment = max(alignment, folio_sz);
> +                                       break;
> +                               }
> +                       }

I think this logic should live in maximum_alignment(), so we don't
have to walk the segments twice. It might be better to move it into a
separate helper, something like should_align_to_exec_folio()?

>
>                         /**
>                          * DOC: PIE handling
>
> > [1] https://sourceware.org/pipermail/libc-alpha/2026-March/175776.html
> > [2] https://lore.kernel.org/linux-fsdevel/20260313005211.882831-1-r@xxxxxx
> >
> > Thanks,
> > Rui
>