https://llvm.org/bugs/show_bug.cgi?id=30293
Bug ID: 30293
Summary: clang-cl miscompilation of Firefox's
netwerk/base/nsSocketTransportService2.cpp
Product: new-bugs
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: froy...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 17216
--> https://llvm.org/bugs/attachment.cgi?id=17216&action=edit
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.
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