Re: [Qemu-devel] Heads up: glibc preadv emulation breaks qemu on older kernels

[Avi Kivity]

On 02/12/2010 07:28 PM, Anthony Liguori wrote:
> On 02/12/2010 07:50 AM, Christoph Hellwig wrote:
> > On Fri, Feb 12, 2010 at 04:49:23PM +0300, malc wrote:
> > > On Fri, 12 Feb 2010, Christoph Hellwig wrote:
> > >
> > > > On Thu, Feb 11, 2010 at 07:06:08PM +0000, Richard W.M. Jones wrote:
> > > > > One interesting thing is that qemu has its own preadv emulation 
> > > > > (which
> > > > > does the emulation correctly), but this is never used because qemu
> > > > > never gets ENOSYS back from preadv.
> > > > At this point the amount of bugs in the glibc preadv/pwritev code 
> > > > really
> > > > make me want to go to use the raw system calls on Linux only.  Any
> > > > opinions from the maintainers if that is acceptable?
> > > There are more than one way to parse the first sentence, if it 
> > > should be
> > > read as:
> > >    On linux and on linux only avoid using pread/write[v] and talk to 
> > > the
> > >    kernel directly.
> > >
> > > Then i agree.
> > Yes.  The BSDs tend to not play stupid emulation games in the libc, so
> > changes of these kinds of messups to happen are far less.
>
> In all fairness, I seem to recall there being a problem with the 
> kernel implementation of preadv/pwritev too.
>
> I think a configure option would be in order.  Forever avoiding glibc 
> is probably a bit extreme.

I'll go further and even avoid the configure options.  Users should run 
fully updated systems.

(how would a user know whether to enable or disable the option? by the 
time they figure out, they might as well update the system).

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.