How to produce deterministic binary output with g++?

Question

I work in a very regulated environment where we need to be able to produce identical binary input give the same source code every time be build out products. We currently use an ancient version of g++ that has been patched to not write anything like a date/time in the resulting binaries that would change from build to build, but I would like to update to g++ 4.7.2. Does anyone know of a patch, or have suggestions of what I need to look for to take two identical pieces of source code and produce identical binary outputs?

Answer 1

We also depend on bit-identical rebuilds, and are using gcc-4.7.x.

Besides setting PWD=/proc/self/cwd and using -frandom-seed=<input-file-name>, there are a handful of patches, which can be found in svn://gcc.gnu.org/svn/gcc/branches/google/gcc-4_7 branch.

Answer 2

The Debian Reproducible builds project attempts to standardize Debian packages byte-by-byte, and has received a Linux Foundation grant in 2016.

While this may include more than compilation, you should have a look at it.

It also pointed me to this article, which adds the following points to what @Employed said:

• put the source in a fixed folder (e.g. /tmp/build) to deal with __FILE__
• for __DATE__, __TIME__, __TIMESTAMP__:
  • libfaketime : https://github.com/wolfcw/libfaketime
  • override those macros with -D
  • -Wdate-time or -Werror=date-time: warn or fail if either __TIME__, __DATE__ or __TIMESTAMP__ are is used. The Linux kernel 4.4 uses it by default.
• use the D flag with ar, or use https://github.com/nh2/ar-timestamp-wiper/tree/master to wipe stamps
• -fno-guess-branch-probability: older manual versions say it is a source of non-determinism, but not anymore. Not sure if this is covered by -frandom-seed or not.

Buildroot has a BR2_REPRODUCIBLE option which may give some ideas on the package level, but it is far from complete at this point.

Related threads:

• https://superuser.com/questions/639351/does-recompiling-a-program-produce-a-bit-for-bit-identical-binary
• https://www.quora.com/What-can-be-the-possible-reasons-for-the-object-code-of-an-unchanged-C-file-to-change-on-recompilation

Answer 3

Use of the 'DATE' macro makes the build non-deterministic