[PATCH] Fix IAT (Import Address Table) alignment on AArch64

[Evgeny Karpov]

On Wed, Apr 08, 2026, Jan Beulich wrote:
> On 08.04.2026 11:27, Evgeny Karpov wrote:
> > On Wed, Apr 08, 2026, Jan Beulich wrote:
> >> On 08.04.2026 11:05, Evgeny Karpov wrote:
> >>> On Wed, Apr 08, 2026, Jan Beulich wrote:
> >>>> On 07.04.2026 19:12, Evgeny Karpov wrote:
> >>>>> On Tue, Apr 07, 2026, Jan Beulich wrote:
> >>>>>> On 07.04.2026 16:47, Evgeny Karpov wrote:
> >>>>>>> On Tue, Apr 07, 2026, Jan Beulich wrote:
> >>>>>>>> On 07.04.2026 11:48, Evgeny Karpov wrote:
> >>>>>>>>> The IAT should be aligned to 8 bytes in aarch64-w64-mingw32, otherwise, it might
> >>>>>>>>> result in relocation issues.
> >>>>>>>>
> >>>>>>>> What is it that makes this different on aarch64? IOW I wonder if this is
> >>>>>>>> going too far or not far enough.
> >>>>>>>>
> >>>>>>>>> binutils/ChangeLog:
> >>>>>>>>>
> >>>>>>>>> 	* dlltool.c (make_head): Update.
> >>>>>>>>
> >>>>>>>> Please either omit the ChangeLog entry, or have it say something meaningful.
> >>>>>>>
> >>>>>>> The updated decription:
> >>>>>>> ---
> >>>>>>> The IAT should be aligned to 8 bytes in aarch64-w64-mingw32, otherwise, it might
> >>>>>>> result in relocation issues.
> >>>>>>>
> >>>>>>> When a function is imported from DLL, it generates the following code:
> >>>>>>>
> >>>>>>> adrp    x19, __imp_fn
> >>>>>>> ldr     x19, [x19, #:lo12:__imp_fn]
> >>>>>>>
> >>>>>>> 8 byte alignment is required for ldr relocation. Addresses are placed in IAT.
> >>>>>>> The size of the chunk on AArch64 is 8 bytes.
> >>>>>>> If IAT is not aligned to 8 bytes, the relocation issue appears.
> >>>>>>> This patch fixes this issue.
> >>>>>>
> >>>>>> Much better, thanks. This addresses part of my earlier remark then as well,
> >>>>>> clarifying it's not "too much" that you do. The "too little" aspect remains,
> >>>>>> though: Looking at a random ia64 archive, I see .idata$5 to have 8-byte
> >>>>>> alignment there as well. Shouldn't .idata$5 always have machine-word
> >>>>>> alignment?
> >>>>>
> >>>>> Based on PE Format documentation
> >>>>> https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#import-address-table
> >>>>>
> >>>>> 4 byte addresses are used for PE32 and 8 byte addresses are used for PE32+.
> >>>>> There is no definition about the alignment, however it might be optional.
> >>>>> 8 byte alignment appears as a requirement for an architecture.
> >>>>> For example, it requires 8 byte alignment for ldr relocation on AArch64.
> >>>>>
> >>>>> Potentially, 8 byte alignment might be applied to all architectures,
> >>>>> however this might not be necessary for other relocations and could
> >>>>> slightly unnecessarily increase the size.
> >>>>
> >>>> Applying 8-byte alignment uniformly might add padding between .idata$4 and
> >>>> .idata$5 for PE32, which likely is unwanted (and possibly is wrong).
> >>>
> >>> Most likely, it does not break the PE format as it has an RVA to IAT, however it is 
> >>> definitely might bring unwanted padding to PE32.
> >>>
> >>>>>>> binutils/ChangeLog:
> >>>>>>>
> >>>>>>>         * dlltool.c (make_head): Add 8 byte alignment.
> >>>>>>
> >>>>>> This sadly still gives the impression of wider a change (all targets) than
> >>>>>> there is.
> >>>>>
> >>>>> It will be changed to "Add 8 byte alignment on AArch64.".
> >>>>
> >>>> If you insist on enforcing the alignment only for Arm64, then the code
> >>>> comment itself also wants extending to clarify why that target is special.
> >>>> As said, I think we should follow what MS tools do and arrange for 4- / 8-
> >>>> byte alignment for all targets.
> >>>
> >>> Based on this suggestion, the patch can be changed to:
> >>>
> >>> +      /* Align IAT (Import Address Table) to 8 bytes for PE32+.
> >>> +        https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#import-address-table.  */
> >>> +      if (create_for_pep)
> >>> +       fprintf (f, "\t.align 3\n");
> >>
> >> Any reason to then not also go the final small step and use
> >>
> >>        fprintf (f, "\t.p2align %d\n", create_for_pep ? 3 : 2);
> > 
> > It looks like the alignment to 4 is not necessary because it is already aligned to
> > 4 by INIT_SEC_DATA (IDATA5, ".idata$5", SEC_HAS_CONTENTS, 2).
> 
> Hmm, good point, yet then wouldn't we better correct things there? And then
> also for .didat? secdata_{plain,delay}[] aren't even const, so a "rude"
> approach could be to edit those tables at runtime. (Preferably we'd solve
> this differently though, e.g. by setting .align to -1 to indicate 2 or 3
> want using depending on the target.)

It might be better to keep default 4 byte alignment for readability.
It turns the change into following:

   create_for_pep = (strcmp (mname, "i386:x86-64") == 0
                    || strcmp (mname, "arm64") == 0);
 
+  if (create_for_pep)
+    {
+      /* Update default 4 byte IAT (Import Address Table) alignemnt to 8 bytes
+        for PE32+.
+        https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#import-address-table.  */
+      secdata[IDATA5].align = 3;
+      secdata_delay[IDATA5].align = 3;
+    }
+

Regards,
Evgeny