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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - clang-cl miscompilation of Firefox's netwerk/base/nsSocketTransportService2.cpp"
href="https://llvm.org/bugs/show_bug.cgi?id=30293">30293</a>
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<th>Summary</th>
<td>clang-cl miscompilation of Firefox's netwerk/base/nsSocketTransportService2.cpp
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>froydnj@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=17216" name="attach_17216" title="preprocessed source and runscript">attachment 17216</a> <a href="attachment.cgi?id=17216&action=edit" title="preprocessed source and runscript">[details]</a></span>
preprocessed source and runscript
While debugging why Firefox crashes when compiled with clang-cl, I ran across
this miscompilation. In nsSocketTransport2.cpp, we have:
nsSocketTransportService* gSocketTransportService = nullptr;
...
void
nsSocketTransportService::OnKeepaliveEnabledPrefChange()
{
// Dispatch to socket thread if we're not executing there.
if (PR_GetCurrentThread() != gSocketThread) {
gSocketTransportService->Dispatch(
NewRunnableMethod(
this, &nsSocketTransportService::OnKeepaliveEnabledPrefChange),
nsIEventTarget::DISPATCH_NORMAL);
return;
}
...
}
The preprocessed source file, along with the command-line flags used to compile
it, are provided in the attached tarball. What I see happening in the debugger
running Firefox is:
1. At the start of OnKeepaliveEnabledPrefChange, ecx contains |this|, and so
does gSocketTransportService.
2. When we load from gSocketTransportService, we adjust its value by 4.
3. When we're preparing to call Dispatch, I think the code is assembling a
member function pointer to be used with some kind of thunk. Whatever it's
doing, the adjusted value we constructed in step (2) winds up in 0(%ecx)
3. When Dispatch() is invoked, we go through what looks like a thunk, but none
of the arguments are massaged in any way; the thunk jumps directly to the
Dispatch() implementation. (I think this is a thunk, anyway; it doesn't appear
in llvm-objdump -d output, but it's definitely there in the debugger, for
reasons I do not understand.)
4. The real Dispatch implementation receives the adjusted pointer from step (3)
in 0(%ecx), acts on it as though it's the actual |this| pointer and things go
south from there. We call nsSocketTransportService::GetThreadSafely() with the
wrong |this| value and it loads a null pointer instead of the actual pointer
it's supposed to load.</pre>
</div>
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