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[PATCH] Facilitate deterministic pe executables between linker invocations

Cory Fields cory@coryfields.com
Wed Oct 9 19:08:00 GMT 2013 
Ping. Just making sure this wasn't lost.

Regards,
Cory

On Tue, Oct 1, 2013 at 1:40 PM, Cory Fields <cory@coryfields.com> wrote:
> I'm not sure if this has been discussed before, or exactly how to propose this
> behavioral change, so I'm submitting this patch with the goal of starting a
> discussion.
>
> It's currently not possible to create two byte-exact exe's due to the timestamp
> in the PE header. This does not match elf behavior, where successive runs can
> produce the exact same binary.
>
> Only a tiny change is needed to avoid the random result. An
> (entirely arbitrary) value of 1 is hard-coded rather than using the current
> timestamp.
>
> Is there a historical reason for the non-deterministic behavior? If so, would
> it be reasonable to add an option similar to enable-deterministic-archives to
> disable it?
>
> Before:
> $ ~/dev/binutils/ld/ld-new -m i386pe -o test.exe <snip>
> $ md5sum test.exe
> d88f78cff7e0f6cf50f4be546c2b4189  test.exe
>
> $ ~/dev/binutils/ld/ld-new -m i386pe -o test.exe <snip>
> $ md5sum test.exe
> 7287892f03f067940b508db830cf85ac  test.exe
>
> After:
> $ ~/dev/binutils/ld/ld-new -m i386pe -o test.exe <snip>
> $ md5sum test.exe
> fa0bf1a326b332f72f270ae060fa758c  test.exe
>
> $ ~/dev/binutils/ld/ld-new -m i386pe -o test.exe <snip>
> $ md5sum test.exe
> fa0bf1a326b332f72f270ae060fa758c  test.exe
>
> binutils/Changelog
> 10-01-2013  Cory Fields  <cory@coryfields.com>
>     * bfd/peXXigen.c (_bfd_XXi_only_swap_filehdr_out): Use a constant rather
>       than a real timestamp in the PE header to ensure deterministic link
>       results when invoked with identical inputs.
> ---
>  bfd/peXXigen.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/bfd/peXXigen.c b/bfd/peXXigen.c
> index d0f7a96..2d9f93c 100644
> --- a/bfd/peXXigen.c
> +++ b/bfd/peXXigen.c
> @@ -793,7 +793,10 @@ _bfd_XXi_only_swap_filehdr_out (bfd * abfd, void * in, void * out)
>    H_PUT_16 (abfd, filehdr_in->f_magic, filehdr_out->f_magic);
>    H_PUT_16 (abfd, filehdr_in->f_nscns, filehdr_out->f_nscns);
>
> -  H_PUT_32 (abfd, time (0), filehdr_out->f_timdat);
> +  /* use a constant for the timestamp to ensure deterministic results with
> +     identical inputs */
> +  H_PUT_32 (abfd, 1, filehdr_out->f_timdat);
> +
>    PUT_FILEHDR_SYMPTR (abfd, filehdr_in->f_symptr,
>                       filehdr_out->f_symptr);
>    H_PUT_32 (abfd, filehdr_in->f_nsyms, filehdr_out->f_nsyms);
> --
> 1.8.1.2
>
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