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Date: Wed, 21 Oct 2009 19:44:14 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH] qemu: work around for "posix-aio-compat"
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>, qemu-devel@nongnu.org


>> I suggest trying to make the sigset_t static, since that generates
>> exactly the same code as the "nohang" case, and exactly the same stack
>> layout as the "hang" case.

(In case this wasn't clear: the sigfillset of a static sigset_t should 
hang, proving that it's stack layout that comes to the rescue).

>> The next obvious step would be placing a
>> watchpoint somewhere.
>
> Yes, but where?

At every word of the sigset (using gdb commands to disable/enable the 
watchpoints around the sigfillset, you avoid spurious triggers).  One of 
those words will be overwritten if an overrun would have smashed the 
stack.  If it does not fire, s/sigfillset/sigemptyset/ in case it was 
writing 0xffffffff.  If it still does not fire, dunno. :-(

Paolo