On Mon, Nov 14, 2011 at 04:30:47PM +0100, Jan Kara wrote:
> On Mon 14-11-11 07:16:26, Matthew Wilcox wrote:
> > On Mon, Nov 14, 2011 at 01:34:46PM +0100, Jan Kara wrote:
> > > On Mon 14-11-11 20:15:56, Wu Fengguang wrote:
> > > > > @@ -2407,6 +2407,10 @@ static ssize_t generic_perform_write(struct file *file,
> > > > > iov_iter_count(i));
> > > > >
> > > > > again:
> > > > > + if (signal_pending(current)) {
> > > >
> > > > signal_pending looks more useful than fatal_signal_pending in that it
> > > > covers normal signals too. However it's exactly the broader coverage
> > > > that makes it an interface change -- will this possibly break casually
> > > > written applications?
> > > Yeah, this is upto discussion. Historically, write() (or any other system
> > > call) could have returned EINTR. In fact, write() to a socket can return
> > > EINTR even now. But you are right that we didn't return EINTR from write()
> > > to a regular file. So if you prefer to never return EINTR from a write to a
> > > regular file, I can change the check since I'm also slightly worried that
> > > some badly written app can notice.
> >
> > No, this is not up for discussion. You can't return short writes (or
> > reads). This is why the 'fatal_signal_pending' API exists -- if the
> > signal is fatal, the task is never returned to, so its bug (not checking
> > the return from read/write) is not exposed.
> By "can't return" you mean userspace need not be expecting it so we
> shouldn't break it or is there some standard which forbids it? Just
> curious...
This *WILL* break userspace code if you return short writes on a
filesystem fd. Guarenteed. I originally wrote code in Samba to
take care of it back before I learned the difference between
"slow" and "fast" interruptable system calls (see W.R.Stevens
for details).
Don't return short writes or reads on a filesystem fd.
Jeremy.
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