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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Clang does not implement the noclone attribute"
href="http://llvm.org/bugs/show_bug.cgi?id=22311">22311</a>
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<th>Summary</th>
<td>Clang does not implement the noclone attribute
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.5
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Driver
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sstewartgallus00@mylangara.bc.ca
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>noclone is a companion attribute to the noinline attribute that GCC implements.
It prevents constant propagation from happening. If one is doing magic low
level hackery then it is a necessity. A specific use case I have for this is
creating a safe wrapper for vfork (which most compilers understandably mess up
on).
The wrapper:
__attribute__((noinline))
__attribute__((noclone))
__attribute__((no_sanitize_address))
static pid_t safe_vfork(int (*f)(void *), void * arg)
{
__atomic_signal_fence(__ATOMIC_SEQ_CST);
pid_t child = vfork();
if (0 == child)
_Exit(f(arg));
return child;
}
Some workarounds:
Use assembly: This is annoying but would work.
Put safe_vfork in a separate module: This doesn't work with link-time
optimization but is the typical workaround.
Use volatile on the function arguments as follows: Strangely, this doesn't
actually work. I can further work around this with more indirection but I am
hesitant to do so because the semantics of volatile are very unclear and
compiler specific.
__attribute__((noinline))
__attribute__((noclone))
__attribute__((no_sanitize_address))
static pid_t safe_vfork(int (*volatile f)(void *), void * volatile arg)
{
__atomic_signal_fence(__ATOMIC_SEQ_CST);
pid_t child = vfork();
if (0 == child)
_Exit(f(arg));
return child;
}</pre>
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