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Date: Thu, 17 Jul 2025 17:36:30 -0700 (PDT)
X-X-Sender: jeremyd AT resin DOT csoft DOT net
To: kikairoya <kikairoya AT gmail DOT com>
cc: cygwin AT cygwin DOT com
Subject: Re: PE format conformance issues in rebase.exe results incompatibility
 with LLVM-lld
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From: Jeremy Drake via Cygwin <cygwin AT cygwin DOT com>
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On Sat, 7 Jun 2025, kikairoya via Cygwin wrote:

> Dear Cygwin team,
>
> I’ve found that rebase.exe makes certain assumptions that do not conform
> to the PE specification.
>
> 1. Incorrect handling of COFF File Header position
> When checking the 'Characteristics' field in the COFF File Header,
> rebase.exe assumes this field is always at a fixed offset. However, the
> PE specification states that the COFF File Header is located immediately
> after the PE signature in the image file.
>
> https://cygwin.com/cgit/cygwin-apps/rebase/tree/rebase.c#n1739
> At this point, the code should perform a relative seek of 18 bytes, not
> an absolute seek with SEEK_SET.
>
> This causes incompatibility with DLLs linked using LLVM's lld, which
> uses a PE stub of a different size. As a result, such DLLs cannot be
> rebased.
>
>
> 2. Limiting file size too conservatively
> https://cygwin.com/cgit/cygwin-apps/rebase/tree/pechecksum.c#n81
>
> Here, the image size is limited to 1 GiB, while the PE specification
> allows for up to 2 GiB.  Large DLLs with debug information (e.g.,
> `cygLLVM*.dll`, `cygclang-cpp*.dll`, etc.) can easily exceed the 1 GiB
> limit.
>
>
> I hope these issues can be addressed to improve compatibility with
> LLVM-based toolchains and enhance the robustness of development
> workflows.


Would you like to make a patch for these?  There may be another issue
because peflags says "checksum update failed" on a small lld-created exe
even when not modifying the binary.

>
> Regards,
>
> --
> Tomohiro Kashiwada (@kikairoya)
>

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