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Re: [libvirt] [PATCH] Ignore SIGWINCH in remote client call to poll(2) (RHBZ#567931).

On Wed, Feb 24, 2010 at 03:39:20PM +0100, Paolo Bonzini wrote:
> On 02/24/2010 02:06 PM, Richard W.M. Jones wrote:
>> The correct solution is to mask out SIGWINCH for the duration
>> of the poll(2) system call.  The per-thread mask is changed and
>> restored immediately after the call.  Since we are using
>> pthread_sigmask, this should not affect other threads, and
>> since we restore the signal mask immediately afterwards it should
>> not affect the current thread visibly either.
> This is interesting.  No signal handler will necessarily do things like  
> longjmp-ing out of the remote driver, so every signal could give rise to  
> a similar bug.

Did you mean "signals handlers might longjmp out ..."?  I've seen a
few in my time that have done that.  I notice the top Google hit for
"signal handler" & "longjmp" is this warning about the practice from


> In particular, SIGCHLD would be an obvious candidate for being handled  
> the same way, since both SIGWINCH and SIGCHLD are default-ignored  
> signals.  On the other hand, while for SIGWINCH it would be mostly  
> harmless(*), for SIGCHLD it would leave a zombie until the remote  
> libvirtd answers.  Any ideas?
>    (*) Unless you have more than one thread using curses, and a thread
>    other than the one calling libvirt has blocked SIGWINCH.

Certainly SIGCHLD could be added.  I was keeping this patch minimal so
it just fixes the problem I observed, to reduce the chance that a
change to such a critical piece of code could break anything else.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://et.redhat.com/~rjones/libguestfs/
See what it can do: http://et.redhat.com/~rjones/libguestfs/recipes.html

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