Archive metadata
Most archive formats record metadata that will capture details about the build environment if no care is taken. File last modification time is obvious, but file ordering, users, groups, numeric ids, and permissions can also be of concern. Tar will be used as the main example but these tips apply to other archive formats as well.
File modification times
Most archive formats will, by default, record file last modification times, while some will also record file creation times.
Tar has a way to specify the modification time that is used for all archive members:
(Notice how Z
is used to specify that time is in the UTC
timezone.)
For other archive formats, it is always possible to use touch
to reset
the modification times to a predefined value
before creating the archive:
In some cases, it is preferable to keep the original times for files that have not been created or modified during the build process:
In tar
>= 1.29, the --clamp-mtime
flag can be used to only set the
when the file is more recent than the value specified with --mtime
:
This has the benefit of leaving the original file modification time untouched.
File ordering
When asked to record directories, most archive formats will read their content in the order returned by the filesystem which is likely to be different on every run.
With version 1.28, GNU Tar has gained the --sort=name
option which will
sort filenames in a locale independent manner:
For older versions or other archive formats, it is possible to use
find
and sort
to achieve the same effect:
Care must be taken to ensure that sort
is called in the context of the
C locale to avoid any surprises related to collation order.
Users, groups and numeric ids
Depending on the archive format, the user and group owning the file can be recorded. Sometimes it will be using a string, sometimes using the associated numeric ids.
When files belong to predefined system groups, this is not a problem, but builds are often performed with regular users. Recording of the account name or its associated ids might be a source of reproducibility issues.
Tar offers a way to specify the user and group owning the file. Using
0
/0
and --numeric-owner
is a safe bet, as it will effectively
record 0 as values:
PAX headers
GNU tar defaults to the pax format and if POSIXLY_CORRECT
is set, that adds files’ ctime, atime and the PID of the tar process as non-deterministic metadata.
To avoid this, either unset POSIXLY_CORRECT
(only works with tar>1.32) or add to the tar call
--pax-option=exthdr.name=%d/PaxHeaders/%f,delete=atime,delete=ctime
or --format=gnu
(both only available in GNU tar)
or use --format=ustar
if the limitations in that format are not a problem.
Full example
The recommended way to create a Tar archive is thus:
Zip files
Zip files can additionally store metadata in “extra file attributes”. We believe these were intended as a cross-platform means of storing, say, Extended Attributes on OS/2 as well as user/group information. Crucially, it can store multiple file timestamps on Unix, including creation, modification and access time. (NB. You may not see access time changes under Linux system is your filesystems are mounted with noatime
or norelatime
).
When creating .zip
files, it is recommended to use the --no-extra
/ -X
argument to not save these fields. It is also recommended that developers unzip archives with TZ=UTC
.
Post-processing
If tools do not support options to create reproducible archives, it is always possible to perform post-processing.
strip-nondeterminism already has support to normalize Zip and Jar archives (with limitations). Custom scripts like Tor Browser’s re-dzip.sh might also be an option.
Static libraries
Static libraries (.a
) on Unix-like systems are ar archives. Like
other archive formats, they contain metadata, namely timestamps, UIDs,
GIDs, and permissions. None are actually required for using them as
libraries.
GNU ar
and other tools from
binutils have a deterministic
mode which will use zero for UIDs, GIDs, timestamps, and use consistent
file modes for all files. It can be made the default by passing the
--enable-deterministic-archives
option to ./configure
. It is already
enabled by default for some distributions1 and so
far it seems to be pretty safe except for
Makefiles using targets like
archive.a(foo.o)
.
When binutils is not built with deterministic archives by default, build
systems have to be changed to pass the right options to ar
and
friends. ARFLAGS
can be set to Dcvr
with many build systems to turn on the
deterministic mode. Care must also be taken to pass -D
if ranlib
is
used to create the function index.
Another option is post-processing with
strip-nondeterminism
or objcopy
:
objcopy --enable-deterministic-archives libfoo.a
The above does not fix file ordering.
Initramfs images
cpio archives are commonly used for initramfs images. The cpio header
format (see man 5 cpio
) can contain device and inode numbers, which whilst
deterministic, can vary from system to system.
One way to filter these is by piping through bsdtar.
Example of non-deterministic code:
echo ucode.bin |
bsdcpio -o -H newc -R 0:0 > ucode.img
Example of deterministic code:
echo ucode.bin |
bsdtar --uid 0 --gid 0 -cnf - -T - |
bsdtar --null -cf - --format=newc @- > ucode.img
Note that other issues such as timestamps may still require rectification prior to archival.
GNU Libtool
GNU Libtool prior to 74c8993c
(first
included in version 2.2.7b) did not sort the find output. It appears that many
packages are bootstrapped with a version prior to this.
Confusingly, although GNU GCC’s ltmain.sh
claims to
have been generated by libtool 2.2.7a, GNU GCC actually maintains their own
version of libtool.m4
and ltmain.sh
, which fixed this issue independently
in d41cd173e23
. This aforementioned change was first included in version
9.1.0, meaning that the reproducibility issue remains in GCC versions below
that.
-
Debian since version 2.25-6/stretch, Ubuntu since version 2.25-8ubuntu1/artful 17.10. It is the default for Fedora 22 and Fedora 23, but it seems this will be reverted in Fedora 24. ↩
Introduction
Achieve deterministic builds
- Commandments of reproducible builds
- Variations in the build environment
- SOURCE_DATE_EPOCH
- Deterministic build systems
- Volatile inputs can disappear
- Stable order for inputs
- Value initialization
- Version information
- Timestamps
- Timezones
- Locales
- Archive metadata
- Stable order for outputs
- Randomness
- Build path
- System images
- JVM
Define a build environment
- What's in a build environment?
- Recording the build environment
- Definition strategies
- Proprietary operating systems