[Reproducible-builds] Bug#819176: gcc-5: Please avoid storing -fdebug-prefix-map in DW_AT_producer (for better reproducibility)

[Daniel Kahn Gillmor]

Package: gcc-5
Version: 5.3.1-12
Severity: wishlist
Tags: patch
User: reproducible-builds at lists.alioth.debian.org
Usertags: buildpath

embedding the build path in binaries makes it harder to get
byte-for-byte reproducibility, because the build has to happen in the
same directory.

gcc by default embeds the build path in the debug info of C tools
(e.g. DW_AT_comp_dir and DW_AT_decl_file).  This isn't useful for
distributed binary packages, because the build environment isn't on
the same machine (or necessarily in the same place).

The build path can be mangled/stripped with gcc's -fdebug-prefix-map
argument.  For example:

   gcc -fdebug-prefix-map=$(pwd)=. -g -o foo foo.c

however, when supplying -fdebug-prefix-map, gcc puts that string
itself into DW_AT_producer.

The attached patch (pulled from upstream gcc master) avoids storing
-fdebug-prefix-map in DW_AT_producer.

I'd like this to be backported into debian's gcc so that we can make
and test a more reproducible toolchain.

(thinking about timing: we shouldn't need the patch once we move to
gcc 6, but we should be testing a buildpath-independent reproducible
toolchain now; i'll file a bug soon to add this pattern to either dh
or dpkg-buildflags)

Regards,

        --dkg

-- System Information:
Debian Release: stretch/sid
Architecture: amd64 (x86_64)

Kernel: Linux 4.4.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages gcc-5 depends on:
ii  binutils      2.26-5
ii  cpp-5         5.3.1-12
ii  gcc-5-base    5.3.1-12
ii  libc6         2.22-3
ii  libcc1-0      5.3.1-12
ii  libgcc-5-dev  5.3.1-12
ii  libgcc1       1:5.3.1-12
ii  libgmp10      2:6.1.0+dfsg-2
ii  libisl15      0.16.1-1
ii  libmpc3       1.0.3-1
ii  libmpfr4      3.1.4-1
ii  libstdc++6    5.3.1-12
ii  zlib1g        1:1.2.8.dfsg-2+b1

Versions of packages gcc-5 recommends:
ii  libc6-dev  2.22-3

Versions of packages gcc-5 suggests:
pn  gcc-5-doc         <none>
pn  gcc-5-locales     <none>
ii  gcc-5-multilib    5.3.1-12
pn  libasan2-dbg      <none>
pn  libatomic1-dbg    <none>
pn  libcilkrts5-dbg   <none>
pn  libgcc1-dbg       <none>
pn  libgomp1-dbg      <none>
pn  libitm1-dbg       <none>
pn  liblsan0-dbg      <none>
pn  libmpx0-dbg       <none>
pn  libquadmath0-dbg  <none>
pn  libtsan0-dbg      <none>
pn  libubsan0-dbg     <none>

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