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[Bug-tar] Getting cross-platform deterministic output with long filename


From: Daniel Peebles
Subject: [Bug-tar] Getting cross-platform deterministic output with long filenames and --numeric-owner
Date: Wed, 5 Apr 2017 14:58:02 -0400

Hi all,

I'm trying to create deterministic tarballs across different systems (Linux and macOS primarily) using GNU tar 1.29. It looks like many people do this successfully with a few options like --owner=0, --group=0, and --numeric-owner (among others like --mtime that aren't relevant to this email).

That usually works fine, but when long filenames come into play, --numeric-owner seems to stop being as effective at suppressing user and group names. Here's a simple example:

touch $(printf 't%.0s' {1..101}) && printf 't%.0s' {1..101} | tar -T - --owner=0 --group=0 --numeric-owner -c | grep root

This should create a simple tar with a single file in it whose name is 101 t characters in a row. Given the --numeric-owner flag I pass in, I'd expect "root" not to appear in the result, but the pipeline above actually does produce a match. And indeed, if I change my /etc/group to map 0 to another name, that name will appear in the output. This starts getting painful on macOS, where group 0 is actually called "wheel".

Poking around at the tar source (https://github.com/Distrotech/tar/blob/release_1_29/src/create.c#L540), I see that theĀ write_gnu_long_link function doesn't seem to pay any attention to my --numeric-owner flags and just blindly inserts the name of the 0 user/group into the LONGLINK struct. Is there a reason for that? It introduces slightly context-sensitive behavior to a process that is otherwise pretty much pure and fully reproducible (with the flags I mention above).

Thanks,
Dan

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