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

[Jan Beulich]

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);

? (Note that you can't use .align when no longer targeting a specific arch;
it needs to be .balign or .p2align.)

Jan

> and the description can be updated with:
> "This patch fixes this issue by aligning IAT to 8 bytes for PE32+." 
> 
> Regards,
> Evgeny