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

Thu Apr 9 13:03:41 GMT 2026

On 09.04.2026 13:21, Evgeny Karpov wrote:
> On Thu, Apr 09, 2026, Jan Beulich wrote:
>> On 09.04.2026 10:10, Evgeny Karpov wrote:
>>> On Wed, Apr 08, 2026, Jan Beulich wrote:
>>>> On 08.04.2026 12:30, Evgeny Karpov wrote:
>>>>> 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;
>>>>
>>>> I guess you mean secdata_plain[] here? Other than that I think all that is
>>>> now needed is a clean v2 submission.
>>>
>>> Actually, applying alignment in secdata_plain is already too late, and it 
>>> should be done in the header for .idata$5 before the first chunk symbol,
>>> as it was done initially.
>>
>> I fear I don't understand: When modifying secdata_{plain,delay}[] right
>> after setting create_for_pep, how can that be too late? gen_lib_file()
>> (and hence make_one_lib_file()) is only called later.
> 
> There is a sequence of .idata$5 sections that starts with a section created
> by using make_head.

Which means what exactly wrt me pointing out that gen_lib_file() (and hence
make_head()) run much later than the editing of the two tables (in main())?

Jan