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<li class="toctree-l1 current"><a class="current reference internal" href="#">5. Frequently asked questions (FAQ)</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#where-did-the-name-charliecloud-come-from">5.1. Where did the name Charliecloud come from?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#how-do-you-spell-charliecloud">5.2. How do you spell Charliecloud?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#my-app-needs-to-write-to-var-log-run-etc">5.3. My app needs to write to <code class="code docutils literal"><span class="pre">/var/log</span></code>, <code class="code docutils literal"><span class="pre">/run</span></code>, etc.</a></li>
<li class="toctree-l2"><a class="reference internal" href="#tarball-build-fails-with-no-command-specified">5.4. Tarball build fails with “No command specified”</a></li>
<li class="toctree-l2"><a class="reference internal" href="#uid-0-lets-me-read-files-i-can-t-otherwise">5.5. <code class="code docutils literal"><span class="pre">--uid</span> <span class="pre">0</span></code> lets me read files I can’t otherwise!</a></li>
<li class="toctree-l2"><a class="reference internal" href="#why-is-bin-being-added-to-my-path">5.6. Why is <code class="code docutils literal"><span class="pre">/bin</span></code> being added to my <code class="code docutils literal"><span class="pre">$PATH</span></code>?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#how-does-setuid-mode-work">5.7. How does setuid mode work?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#ch-run-fails-with-can-t-re-mount-image-read-only">5.8. <code class="code docutils literal"><span class="pre">ch-run</span></code> fails with “can’t re-mount image read-only”</a></li>
<li class="toctree-l2"><a class="reference internal" href="#which-specific-sudo-commands-are-needed">5.9. Which specific <code class="code docutils literal"><span class="pre">sudo</span></code> commands are needed?</a></li>
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<div class="section" id="frequently-asked-questions-faq">
<h1>5. Frequently asked questions (FAQ)<a class="headerlink" href="#frequently-asked-questions-faq" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#where-did-the-name-charliecloud-come-from" id="id1">Where did the name Charliecloud come from?</a></li>
<li><a class="reference internal" href="#how-do-you-spell-charliecloud" id="id2">How do you spell Charliecloud?</a></li>
<li><a class="reference internal" href="#my-app-needs-to-write-to-var-log-run-etc" id="id3">My app needs to write to <code class="code docutils literal"><span class="pre">/var/log</span></code>, <code class="code docutils literal"><span class="pre">/run</span></code>, etc.</a></li>
<li><a class="reference internal" href="#tarball-build-fails-with-no-command-specified" id="id4">Tarball build fails with “No command specified”</a></li>
<li><a class="reference internal" href="#uid-0-lets-me-read-files-i-can-t-otherwise" id="id5"><code class="code docutils literal"><span class="pre">--uid</span> <span class="pre">0</span></code> lets me read files I can’t otherwise!</a></li>
<li><a class="reference internal" href="#why-is-bin-being-added-to-my-path" id="id6">Why is <code class="code docutils literal"><span class="pre">/bin</span></code> being added to my <code class="code docutils literal"><span class="pre">$PATH</span></code>?</a></li>
<li><a class="reference internal" href="#how-does-setuid-mode-work" id="id7">How does setuid mode work?</a></li>
<li><a class="reference internal" href="#ch-run-fails-with-can-t-re-mount-image-read-only" id="id8"><code class="code docutils literal"><span class="pre">ch-run</span></code> fails with “can’t re-mount image read-only”</a></li>
<li><a class="reference internal" href="#which-specific-sudo-commands-are-needed" id="id9">Which specific <code class="code docutils literal"><span class="pre">sudo</span></code> commands are needed?</a></li>
</ul>
</div>
<div class="section" id="where-did-the-name-charliecloud-come-from">
<h2><a class="toc-backref" href="#id1">5.1. Where did the name Charliecloud come from?</a><a class="headerlink" href="#where-did-the-name-charliecloud-come-from" title="Permalink to this headline">¶</a></h2>
<p><em>Charlie</em> — Charles F. McMillan was director of Los Alamos National Laboratory
from June 2011 until December 2017, i.e., at the time Charliecloud was started
in early 2014. He is universally referred to as “Charlie” here.</p>
<p><em>cloud</em> — Charliecloud provides cloud-like flexibility for HPC systems.</p>
</div>
<div class="section" id="how-do-you-spell-charliecloud">
<h2><a class="toc-backref" href="#id2">5.2. How do you spell Charliecloud?</a><a class="headerlink" href="#how-do-you-spell-charliecloud" title="Permalink to this headline">¶</a></h2>
<p>We try to be consistent with <em>Charliecloud</em> — one word, no camel case. That
is, <em>Charlie Cloud</em> and <em>CharlieCloud</em> are both incorrect.</p>
</div>
<div class="section" id="my-app-needs-to-write-to-var-log-run-etc">
<h2><a class="toc-backref" href="#id3">5.3. My app needs to write to <code class="code docutils literal"><span class="pre">/var/log</span></code>, <code class="code docutils literal"><span class="pre">/run</span></code>, etc.</a><a class="headerlink" href="#my-app-needs-to-write-to-var-log-run-etc" title="Permalink to this headline">¶</a></h2>
<p>Because the image is mounted read-only by default, log files, caches, and
other stuff cannot be written anywhere in the image. You have three options:</p>
<ol class="arabic simple">
<li>Configure the application to use a different directory. <code class="code docutils literal"><span class="pre">/tmp</span></code> is
often a good choice, because it’s shared with the host and fast.</li>
<li>Use <code class="code docutils literal"><span class="pre">RUN</span></code> commands in your Dockerfile to create symlinks that point
somewhere writeable, e.g. <code class="code docutils literal"><span class="pre">/tmp</span></code>, or <code class="code docutils literal"><span class="pre">/mnt/0</span></code> with
<code class="code docutils literal"><span class="pre">ch-run</span> <span class="pre">--bind</span></code>.</li>
<li>Run the image read-write with <code class="code docutils literal"><span class="pre">ch-run</span> <span class="pre">-w</span></code>. Be careful that multiple
containers do not try to write to the same image files.</li>
</ol>
</div>
<div class="section" id="tarball-build-fails-with-no-command-specified">
<h2><a class="toc-backref" href="#id4">5.4. Tarball build fails with “No command specified”</a><a class="headerlink" href="#tarball-build-fails-with-no-command-specified" title="Permalink to this headline">¶</a></h2>
<p>The full error from <code class="code docutils literal"><span class="pre">ch-docker2tar</span></code> or <code class="code docutils literal"><span class="pre">ch-build2dir</span></code> is:</p>
<div class="highlight-console"><div class="highlight"><pre><span></span><span class="go">docker: Error response from daemon: No command specified.</span>
</pre></div>
</div>
<p>You will also see it with various plain Docker commands.</p>
<p>This happens when there is no default command specified in the Dockerfile or
any of its ancestors. Some base images specify one (e.g., Debian) and others
don’t (e.g., Alpine). Docker requires this even for commands that don’t seem
like they should need it, such as <code class="code docutils literal"><span class="pre">docker</span> <span class="pre">create</span></code> (which is what trips
up Charliecloud).</p>
<p>The solution is to add a default command to your Dockerfile, such as
<code class="code docutils literal"><span class="pre">CMD</span> <span class="pre">["true"]</span></code>.</p>
</div>
<div class="section" id="uid-0-lets-me-read-files-i-can-t-otherwise">
<h2><a class="toc-backref" href="#id5">5.5. <code class="code docutils literal"><span class="pre">--uid</span> <span class="pre">0</span></code> lets me read files I can’t otherwise!</a><a class="headerlink" href="#uid-0-lets-me-read-files-i-can-t-otherwise" title="Permalink to this headline">¶</a></h2>
<p>Some permission bits can give a surprising result with a container UID of 0.
For example:</p>
<div class="highlight-console"><div class="highlight"><pre><span></span><span class="gp">$</span> whoami
<span class="go">reidpr</span>
<span class="gp">$</span> <span class="nb">echo</span> surprise > ~/cantreadme
<span class="gp">$</span> chmod <span class="m">000</span> ~/cantreadme
<span class="gp">$</span> ls -l ~/cantreadme
<span class="go">---------- 1 reidpr reidpr 9 Oct 3 15:03 /home/reidpr/cantreadme</span>
<span class="gp">$</span> cat ~/cantreadme
<span class="go">cat: /home/reidpr/cantreadme: Permission denied</span>
<span class="gp">$</span> ch-run /var/tmp/hello cat ~/cantreadme
<span class="go">cat: /home/reidpr/cantreadme: Permission denied</span>
<span class="gp">$</span> ch-run --uid <span class="m">0</span> /var/tmp/hello cat ~/cantreadme
<span class="go">surprise</span>
</pre></div>
</div>
<p>At first glance, it seems that we’ve found an escalation – we were able to
read a file inside a container that we could not read on the host! That seems
bad.</p>
<p>However, what is really going on here is more prosaic but complicated:</p>
<ol class="arabic simple">
<li>After <code class="code docutils literal"><span class="pre">unshare(CLONE_NEWUSER)</span></code>, <code class="code docutils literal"><span class="pre">ch-run</span></code> gains all capabilities
inside the namespace. (Outside, capabilities are unchanged.)</li>
<li>This include <code class="code docutils literal"><span class="pre">CAP_DAC_OVERRIDE</span></code>, which enables a process to
read/write/execute a file or directory mostly regardless of its permission
bits. (This is why root isn’t limited by permissions.)</li>
<li>Within the container, <code class="code docutils literal"><span class="pre">exec(2)</span></code> capability rules are followed.
Normally, this basically means that all capabilities are dropped when
<code class="code docutils literal"><span class="pre">ch-run</span></code> replaces itself with the user command. However, if EUID is
0, which it is inside the namespace given <code class="code docutils literal"><span class="pre">--uid</span> <span class="pre">0</span></code>, then the
subprocess keeps all its capabilities. (This makes sense: if root creates a
new process, it stays root.)</li>
<li><code class="code docutils literal"><span class="pre">CAP_DAC_OVERRIDE</span></code> within a user namespace is honored for a file or
directory only if its UID and GID are both mapped. In this case,
<code class="code docutils literal"><span class="pre">ch-run</span></code> maps <code class="code docutils literal"><span class="pre">reidpr</span></code> to container <code class="code docutils literal"><span class="pre">root</span></code> and group
<code class="code docutils literal"><span class="pre">reidpr</span></code> to itself.</li>
<li>Thus, files and directories owned by the host EUID and EGID (here
<code class="code docutils literal"><span class="pre">reidpr:reidpr</span></code>) are available for all access with <code class="code docutils literal"><span class="pre">ch-run</span>
<span class="pre">--uid</span> <span class="pre">0</span></code>.</li>
</ol>
<p>This isn’t a problem. The quirk applies only to files owned by the invoking
user, because <code class="code docutils literal"><span class="pre">ch-run</span></code> is unprivileged outside the namespace, and thus
he or she could simply <code class="code docutils literal"><span class="pre">chmod</span></code> the file to read it. Access inside and
outside the container remains equivalent.</p>
<p>References:</p>
<ul class="simple">
<li><a class="reference external" href="http://man7.org/linux/man-pages/man7/capabilities.7.html">http://man7.org/linux/man-pages/man7/capabilities.7.html</a></li>
<li><a class="reference external" href="http://lxr.free-electrons.com/source/kernel/capability.c?v=4.2#L442">http://lxr.free-electrons.com/source/kernel/capability.c?v=4.2#L442</a></li>
<li><a class="reference external" href="http://lxr.free-electrons.com/source/fs/namei.c?v=4.2#L328">http://lxr.free-electrons.com/source/fs/namei.c?v=4.2#L328</a></li>
</ul>
</div>
<div class="section" id="why-is-bin-being-added-to-my-path">
<h2><a class="toc-backref" href="#id6">5.6. Why is <code class="code docutils literal"><span class="pre">/bin</span></code> being added to my <code class="code docutils literal"><span class="pre">$PATH</span></code>?</a><a class="headerlink" href="#why-is-bin-being-added-to-my-path" title="Permalink to this headline">¶</a></h2>
<p>Newer Linux distributions replace some root-level directories, such as
<code class="code docutils literal"><span class="pre">/bin</span></code>, with symlinks to their counterparts in <code class="code docutils literal"><span class="pre">/usr</span></code>.</p>
<p>Some of these distributions (e.g., Fedora 24) have also dropped <code class="code docutils literal"><span class="pre">/bin</span></code>
from the default <code class="code docutils literal"><span class="pre">$PATH</span></code>. This is a problem when the guest OS does <em>not</em>
have a merged <code class="code docutils literal"><span class="pre">/usr</span></code> (e.g., Debian 8 “Jessie”).</p>
<p>While Charliecloud’s general philosophy is not to manipulate environment
variables, in this case, guests can be severely broken if <code class="code docutils literal"><span class="pre">/bin</span></code> is not
in <code class="code docutils literal"><span class="pre">$PATH</span></code>. Thus, we add it if it’s not there.</p>
<p>Further reading:</p>
<blockquote>
<div><ul class="simple">
<li><a class="reference external" href="https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/">The case for the /usr Merge</a></li>
<li><a class="reference external" href="https://fedoraproject.org/wiki/Features/UsrMove">Fedora</a></li>
<li><a class="reference external" href="https://wiki.debian.org/UsrMerge">Debian</a></li>
</ul>
</div></blockquote>
</div>
<div class="section" id="how-does-setuid-mode-work">
<h2><a class="toc-backref" href="#id7">5.7. How does setuid mode work?</a><a class="headerlink" href="#how-does-setuid-mode-work" title="Permalink to this headline">¶</a></h2>
<p>As noted above, <code class="code docutils literal"><span class="pre">ch-run</span></code> has a transition mode that uses setuid-root
privileges instead of user namespaces. The goal of this mode is to let sites
evaluate Charliecloud even on systems that do not have a Linux kernel that
supports user namespaces. We plan to remove this code once user namespaces are
more widely available, and we encourage sites to use the unprivileged,
non-setuid mode in production.</p>
<p>We haven taken care to (1) drop privileges temporarily upon program start and
only re-acquire them when needed and (2) drop privileges permanently before
executing user code. In order to reliably verify the latter, <code class="code docutils literal"><span class="pre">ch-run</span></code> in
setuid mode will refuse to run if invoked directly by root.</p>
<p>It may be better to use capabilities and setcap rather than setuid. However,
this also relies on newer features, which would hamper the goal of broadly
available testing. For example, NFSv3 does not support extended attributes,
which are required for setcap files.</p>
<p>Dropping privileges safely requires care. We follow the recommendations in
“<a class="reference external" href="https://www.usenix.org/legacy/events/sec02/full_papers/chen/chen.pdf">Setuid demystified</a>” as
well as the <a class="reference external" href="https://www.securecoding.cert.org/confluence/display/c/POS36-C.+Observe+correct+revocation+order+while+relinquishing+privileges">system call ordering</a>
and <a class="reference external" href="https://www.securecoding.cert.org/confluence/display/c/POS37-C.+Ensure+that+privilege+relinquishment+is+successful">privilege drop verification</a>
recommendations of the SEI CERT C Coding Standard.</p>
<p>We do not worry about the Linux-specific <code class="code docutils literal"><span class="pre">fsuid</span></code> and <code class="code docutils literal"><span class="pre">fsgid</span></code>,
which track <code class="code docutils literal"><span class="pre">euid</span></code>/<code class="code docutils literal"><span class="pre">egid</span></code> unless specifically changed, which we
don’t do. Kernel bugs have existed that violate this invariant, but none are
recent.</p>
</div>
<div class="section" id="ch-run-fails-with-can-t-re-mount-image-read-only">
<h2><a class="toc-backref" href="#id8">5.8. <code class="code docutils literal"><span class="pre">ch-run</span></code> fails with “can’t re-mount image read-only”</a><a class="headerlink" href="#ch-run-fails-with-can-t-re-mount-image-read-only" title="Permalink to this headline">¶</a></h2>
<p>Normally, <code class="code docutils literal"><span class="pre">ch-run</span></code> re-mounts the image directory read-only within the
container. This fails if the image resides on certain filesystems, such as NFS
(see <a class="reference external" href="https://github.com/hpc/charliecloud/issues/9">issue #9</a>). There are
two solutions:</p>
<ol class="arabic simple">
<li>Unpack the image into a different filesystem, such as <code class="code docutils literal"><span class="pre">tmpfs</span></code> or
local disk. Consult your local admins for a recommendation. Note that
<code class="code docutils literal"><span class="pre">tmpfs</span></code> is a lot faster than Lustre.</li>
<li>Use the <code class="code docutils literal"><span class="pre">-w</span></code> switch to leave the image mounted read-write. Note that
this has may have an impact on reproducibility (because the application can
change the image between runs) and/or stability (if there are multiple
application processes and one writes a file in the image that another is
reading or writing).</li>
</ol>
</div>
<div class="section" id="which-specific-sudo-commands-are-needed">
<h2><a class="toc-backref" href="#id9">5.9. Which specific <code class="code docutils literal"><span class="pre">sudo</span></code> commands are needed?</a><a class="headerlink" href="#which-specific-sudo-commands-are-needed" title="Permalink to this headline">¶</a></h2>
<p>For running images, <code class="code docutils literal"><span class="pre">sudo</span></code> is not needed at all.</p>
<p>For building images, it depends on what you would like to support. For
example, do you want to let users build images with Docker? Do you want to let
them run the build tests?</p>
<p>We do not maintain specific lists, but you can search the source code and
documentation for uses of <code class="code docutils literal"><span class="pre">sudo</span></code> and <code class="code docutils literal"><span class="pre">$DOCKER</span></code> and evaluate them
on a case-by-case basis. (The latter includes <code class="code docutils literal"><span class="pre">sudo</span></code> if needed to invoke
<code class="code docutils literal"><span class="pre">docker</span></code> in your environment.) For example:</p>
<div class="highlight-console"><div class="highlight"><pre><span></span><span class="gp">$</span> find . <span class="se">\(</span> -type f -executable <span class="se">\</span>
-o -name Makefile <span class="se">\</span>
-o -name <span class="s1">'*.bats'</span> <span class="se">\</span>
-o -name <span class="s1">'*.rst'</span> <span class="se">\</span>
-o -name <span class="s1">'*.sh'</span> <span class="se">\)</span> <span class="se">\</span>
-exec egrep -H <span class="s1">'(sudo|\$DOCKER)'</span> <span class="o">{}</span> <span class="se">\;</span>
</pre></div>
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