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  <section id="how-varnish-met-cheri-5-n">
<span id="phk-cheri-5"></span><h1>How Varnish met CHERI 5/N<a class="headerlink" href="#how-varnish-met-cheri-5-n" title="Link to this heading">¶</a></h1>
<section id="varnish-workspaces">
<h2>Varnish Workspaces<a class="headerlink" href="#varnish-workspaces" title="Link to this heading">¶</a></h2>
<p>To process a HTTP request or response, varnish must allocate bits
of memory which will only be used for the duration of that processing,
and all of it can be released back at the same time.</p>
<p>To avoid calling <code class="docutils literal notranslate"><span class="pre">malloc(3)</span></code> a lot, which comes with a locking
overhead in a heavily multithreaded process, but even more to
avoid having to keep track of all these allocations in order to be able
to <code class="docutils literal notranslate"><span class="pre">free(3)</span></code> them all, varnish has “workspaces”:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>struct ws {
    […]
    char    *s;     /* (S)tart of buffer */
    char    *f;     /* (F)ree/front pointer */
    char    *r;     /* (R)eserved length */
    char    *e;     /* (E)nd of buffer */
};
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">s</span></code> pointer points at the start of a slab of memory, owned
exclusively by the current thread and <code class="docutils literal notranslate"><span class="pre">e</span></code> points to the end.</p>
<p>Initially <code class="docutils literal notranslate"><span class="pre">f</span></code> is the same as <code class="docutils literal notranslate"><span class="pre">s</span></code>, but as allocations are made
from the workspace, it moves towards <code class="docutils literal notranslate"><span class="pre">e</span></code>.  The <code class="docutils literal notranslate"><span class="pre">r</span></code> pointer is
used to make “reservations”, we will ignore that for now.</p>
<p>Workspaces look easy to create:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>ws-&gt;s = space;
ws-&gt;e = ws-&gt;s + len;
ws-&gt;f = ws-&gt;s;
ws-&gt;r = NULL;
</pre></div>
</div>
<p>… only, given the foot-shooting-abetting nature of the C language,
we have bolted on a lot of seat-belts:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>#define WS_ID_SIZE 4

struct ws {
    unsigned        magic;
#define WS_MAGIC    0x35fac554
    char            id[WS_ID_SIZE]; /* identity */
    char            *s;             /* (S)tart of buffer */
    char            *f;             /* (F)ree/front pointer */
    char            *r;             /* (R)eserved length */
    char            *e;             /* (E)nd of buffer */
};

void
WS_Init(struct ws *ws, const char *id, void *space, unsigned len)
{
    unsigned l;

DSLb(DBG_WORKSPACE,
        &quot;WS_Init(%s, %p, %p, %u)&quot;, id, ws, space, len);
    assert(space != NULL);
    assert(PAOK(space));
    INIT_OBJ(ws, WS_MAGIC);
    ws-&gt;s = space;
    l = PRNDDN(len - 1);
    ws-&gt;e = ws-&gt;s + l;
    memset(ws-&gt;e, WS_REDZONE_END, len - l);
    ws-&gt;f = ws-&gt;s;
    assert(id[0] &amp; 0x20);           // cheesy islower()
    bstrcpy(ws-&gt;id, id);
    WS_Assert(ws);
}
</pre></div>
</div>
<p>Let me walk you through that:</p>
<p>The <code class="docutils literal notranslate"><span class="pre">DSLb()</span></code> call can be used to trace all operations on the
workspace, so we can see what actually goes on.</p>
<p>(Hint: Your <code class="docutils literal notranslate"><span class="pre">malloc(3)</span></code> may have something similar,
look for <code class="docutils literal notranslate"><span class="pre">utrace</span></code> in the manual page.)</p>
<p>Next we check the provided space pointer is not NULL, and
that it is properly aligned, these are both following
a varnish style-pattern, to sprinkle asserts liberally,
both as code documentation, but also because it allows
the compiler to optimize things better.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">INIT_OBJ()</span> <span class="pre">and</span> <span class="pre">``magic</span></code> field is a style-pattern
we use throughout varnish:  Each structure is tagged with
a unique magic, which can be used to ensure that pointers
are what we are told, when they get passed through a <code class="docutils literal notranslate"><span class="pre">void*</span></code>.</p>
<p>We set the <code class="docutils literal notranslate"><span class="pre">s</span></code> pointer.</p>
<p>We calculate a length at least one byte shorter than what
we were provided, align it, and point <code class="docutils literal notranslate"><span class="pre">e</span></code> at that.</p>
<p>We fill that extraspace at and past <code class="docutils literal notranslate"><span class="pre">e</span></code>, with a “canary” to
stochastically detect overruns.  It catches most but not
all overruns.</p>
<p>We set the name of the workspace, ensuring it is not already
marked as overflowed.</p>
<p>And finally check that the resulting workspace complies with
the defined invariants, as captured in the <code class="docutils literal notranslate"><span class="pre">WS_Assert()</span></code>
function.</p>
<p>With CHERI, it looks like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>void
WS_Init(struct ws *ws, const char *id, void *space, unsigned len)
{
    unsigned l;

DSLb(DBG_WORKSPACE,
        &quot;WS_Init(%s, %p, %p, %u)&quot;, id, ws, space, len);
    assert(space != NULL);
    INIT_OBJ(ws, WS_MAGIC);
    assert(PAOK(space));
    ws-&gt;s = cheri_bounds_set(space, len);
    ws-&gt;e = ws-&gt;s + len
    ws-&gt;f = ws-&gt;s;
    assert(id[0] &amp; 0x20);           // cheesy islower()
    bstrcpy(ws-&gt;id, id);
    WS_Assert(ws);
}
</pre></div>
</div>
<p>All the gunk to implement a canary to detect overruns went
away, because with CHERI we can restrict the <code class="docutils literal notranslate"><span class="pre">s</span></code> pointer so writing
outside the workspace is <em>by definition</em> impossible, as long as your
pointer is derived from <code class="docutils literal notranslate"><span class="pre">s</span></code>.</p>
<p>Less memory wasted, much stronger check and more readable source-code,
what’s not to like ?</p>
<p>When an allocation is made from the workspace, CHERI makes it possible
to restrict the returned pointer to just the allocated space:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>void *
WS_Alloc(struct ws *ws, unsigned bytes)
{
    char *r;

[…]
    r = ws-&gt;f;
    ws-&gt;f += bytes;
    return(cheri_bounds_set(r, bytes));
}
</pre></div>
</div>
</section>
<section id="varnish-string-buffers">
<h2>Varnish String Buffers<a class="headerlink" href="#varnish-string-buffers" title="Link to this heading">¶</a></h2>
<p>Back in the mists of time, Dag-Erling Smørgrav and I designed a
safe string API called <code class="docutils literal notranslate"><span class="pre">sbuf</span></code> for the FreeBSD kernel.</p>
<p>The basic idea is you set up your buffer, you call functions to stuff
text into it, and those functions do all the hard work to ensure
you do not overrun the buffer.  When the string is complete, you
call a function to “finish” the buffer, and if returns a flag which
tells you if overrun (or other problems) happened, and then you can
get a pointer to the resulting string from another function.</p>
<p>Varnish has adopted sbuf’s under the name <code class="docutils literal notranslate"><span class="pre">vsb</span></code>.  This should
really not surprise anybody:  Dag-Erling was also involved
in the birth of varnish.</p>
<p>It should be obvious that internally <code class="docutils literal notranslate"><span class="pre">vsb</span></code> almost always operate
on a bigger buffer than the result, so this is another obvious
place to have CHERI cut a pointer down to size:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>char *
VSB_data(const struct vsb *s)
{

assert_VSB_integrity(s);
    assert_VSB_state(s, VSB_FINISHED);

return (cheri_bounds_set(s-&gt;s_buf, s-&gt;s_len + 1));
}
</pre></div>
</div>
<p>Still no bugs though.</p>
<p><em>/phk</em></p>
</section>
</section>

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    <ul>
<li><a class="reference internal" href="#">How Varnish met CHERI 5/N</a><ul>
<li><a class="reference internal" href="#varnish-workspaces">Varnish Workspaces</a></li>
<li><a class="reference internal" href="#varnish-string-buffers">Varnish String Buffers</a></li>
</ul>
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