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

[Usama Arif <usama.arif@linux.dev>]

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;
+                               }
+                       }
 
                        /**
                         * 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@hev.cc
> 
> Thanks,
> Rui