[PATCH] Facilitate deterministic pe executables between linker invocations

[Cory Fields <cory@coryfields.com>]

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