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diff --git a/doc/guix.texi b/doc/guix.texi index 6d24343025..3222a64085 100644 --- a/doc/guix.texi +++ b/doc/guix.texi @@ -143,6 +143,7 @@ Utilities * Invoking guix graph:: Visualizing the graph of packages. * Invoking guix environment:: Setting up development environments. * Invoking guix publish:: Sharing substitutes. +* Invoking guix challenge:: Challenging substitute servers. GNU Distribution @@ -1600,7 +1601,10 @@ Guix has the foundations to maximize build reproducibility (@pxref{Features}). In most cases, independent builds of a given package or derivation should yield bit-identical results. Thus, through a diverse set of independent package builds, we can strengthen the -integrity of our systems. +integrity of our systems. The @command{guix challenge} command aims to +help users assess substitute servers, and to assist developers in +finding out about non-deterministic package builds (@pxref{Invoking guix +challenge}). In the future, we want Guix to have support to publish and retrieve binaries to/from other users, in a peer-to-peer fashion. If you would @@ -3577,6 +3581,7 @@ programming interface of Guix in a convenient way. * Invoking guix graph:: Visualizing the graph of packages. * Invoking guix environment:: Setting up development environments. * Invoking guix publish:: Sharing substitutes. +* Invoking guix challenge:: Challenging substitute servers. @end menu @node Invoking guix build @@ -4776,6 +4781,128 @@ Reference Manual}) on @var{port} (37146 by default). This is used primarily for debugging a running @command{guix publish} server. @end table + +@node Invoking guix challenge +@section Invoking @command{guix challenge} + +@cindex reproducible builds +@cindex verifiable builds + +Do the binaries provided by this server really correspond to the source +code it claims to build? Is this package's build process deterministic? +These are the questions the @command{guix challenge} command attempts to +answer. + +The former is obviously an important question: Before using a substitute +server (@pxref{Substitutes}), you'd rather @emph{verify} that it +provides the right binaries, and thus @emph{challenge} it. The latter +is what enables the former: If package builds are deterministic, then +independent builds of the package should yield the exact same result, +bit for bit; if a server provides a binary different from the one +obtained locally, it may be either corrupt or malicious. + +We know that the hash that shows up in @file{/gnu/store} file names is +the hash of all the inputs of the process that built the file or +directory---compilers, libraries, build scripts, +etc. (@pxref{Introduction}). Assuming deterministic build processes, +one store file name should map to exactly one build output. +@command{guix challenge} checks whether there is, indeed, a single +mapping by comparing the build outputs of several independent builds of +any given store item. + +The command's output looks like this: + +@smallexample +$ guix challenge --substitute-urls="http://hydra.gnu.org http://guix.example.org" +updating list of substitutes from 'http://hydra.gnu.org'... 100.0% +updating list of substitutes from 'http://guix.example.org'... 100.0% +/gnu/store/@dots{}-openssl-1.0.2d contents differ: + local hash: 0725l22r5jnzazaacncwsvp9kgf42266ayyp814v7djxs7nk963q + http://hydra.gnu.org/nar/@dots{}-openssl-1.0.2d: 0725l22r5jnzazaacncwsvp9kgf42266ayyp814v7djxs7nk963q + http://guix.example.org/nar/@dots{}-openssl-1.0.2d: 1zy4fmaaqcnjrzzajkdn3f5gmjk754b43qkq47llbyak9z0qjyim +/gnu/store/@dots{}-git-2.5.0 contents differ: + local hash: 00p3bmryhjxrhpn2gxs2fy0a15lnip05l97205pgbk5ra395hyha + http://hydra.gnu.org/nar/@dots{}-git-2.5.0: 069nb85bv4d4a6slrwjdy8v1cn4cwspm3kdbmyb81d6zckj3nq9f + http://guix.example.org/nar/@dots{}-git-2.5.0: 0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73 +/gnu/store/@dots{}-pius-2.1.1 contents differ: + local hash: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax + http://hydra.gnu.org/nar/@dots{}-pius-2.1.1: 0k4v3m9z1zp8xzzizb7d8kjj72f9172xv078sq4wl73vnq9ig3ax + http://guix.example.org/nar/@dots{}-pius-2.1.1: 1cy25x1a4fzq5rk0pmvc8xhwyffnqz95h2bpvqsz2mpvlbccy0gs +@end smallexample + +@noindent +In this example, @command{guix challenge} first scans the store to +determine the set of locally-built derivations---as opposed to store +items that were downloaded from a substitute server---and then queries +all the substitute servers. It then reports those store items for which +the servers obtained a result different from the local build. + +@cindex non-determinism, in package builds +As an example, @code{guix.example.org} always gets a different answer. +Conversely, @code{hydra.gnu.org} agrees with local builds, except in the +case of Git. This might indicate that the build process of Git is +non-deterministic, meaning that its output varies as a function of +various things that Guix does not fully control, in spite of building +packages in isolated environments (@pxref{Features}). Most common +sources of non-determinism include the addition of timestamps in build +results, the inclusion of random numbers, and directory listings sorted +by inode number. See @uref{http://reproducible.debian.net/howto/}, for +more information. + +To find out what's wrong with this Git binary, we can do something along +these lines (@pxref{Invoking guix archive}): + +@example +$ wget -q -O - http://hydra.gnu.org/nar/@dots{}-git-2.5.0 \ + | guix archive -x /tmp/git +$ diff -ur /gnu/store/@dots{}-git.2.5.0 /tmp/git +@end example + +This command shows the difference between the files resulting from the +local build, and the files resulting from the build on +@code{hydra.gnu.org} (@pxref{Overview, Comparing and Merging Files,, +diffutils, Comparing and Merging Files}). The @command{diff} command +works great for text files. When binary files differ, a better option +is @uref{http://diffoscope.org/, Diffoscope}, a tool that helps +visualize differences for all kinds of files. + +Once you've done that work, you can tell whether the differences are due +to a non-deterministic build process or to a malicious server. We try +hard to remove sources of non-determinism in packages to make it easier +to verify substitutes, but of course, this is a process, one that +involves not just Guix but a large part of the free software community. +In the meantime, @command{guix challenge} is one tool to help address +the problem. + +If you are writing packages for Guix, you are encouraged to check +whether @code{hydra.gnu.org} and other substitute servers obtain the +same build result as you did with: + +@example +$ guix challenge @var{package} +@end example + +@noindent +... where @var{package} is a package specification such as +@code{guile-2.0} or @code{glibc:debug}. + +The general syntax is: + +@example +guix challenge @var{options} [@var{packages}@dots{}] +@end example + +The one option that matters is: + +@table @code + +@item --substitute-urls=@var{urls} +Consider @var{urls} the whitespace-separated list of substitute source +URLs to compare to. + +@end table + + @c ********************************************************************* @node GNU Distribution @chapter GNU Distribution |