Re: [UPDATE PATCH][Bug 5132] fix sys_poll() large timeout handling

[Peter Staubach]

Nishanth Aravamudan wrote:

> On 09.09.2005 [19:36:21 -0700], Andrew Morton wrote:
>  
>
> > Nishanth Aravamudan <nacc@xxxxxxxxxx> wrote:
> >    
> >
> > > +	/*
> > > +	 * We compare HZ with 1000 to work out which side of the
> > > +	 * expression needs conversion.  Because we want to avoid
> > > +	 * converting any value to a numerically higher value, which
> > > +	 * could overflow.
> > > +	 */
> > > +#if HZ > 1000
> > > +	overflow = timeout_msecs >= jiffies_to_msecs(MAX_SCHEDULE_TIMEOUT);
> > > +#else
> > > +	overflow = msecs_to_jiffies(timeout_msecs) >= MAX_SCHEDULE_TIMEOUT;
> > > +#endif
> > > +
> > > +	/*
> > > +	 * If we would overflow in the conversion or a negative timeout
> > > +	 * is requested, sleep indefinitely.
> > > +	 */
> > > +	if (overflow || timeout_msecs < 0)
> > > +		timeout_jiffies = MAX_SCHEDULE_TIMEOUT;
> > >      
> > Do we need to test (timeout_msecs < 0) here?  If we make timeout_msecs
> > unsigned long then I think `overflow' will always be correct.
> >    
>
> Even though poll is explicitly allowed to take negative values, as per
> my man-page:
>
> "#include <sys/poll.h>
>
> int poll(struct pollfd *ufds, unsigned int nfds, int timeout);
>
> ...
>
> A negative value means infinite timeout."
>
> Would we have a local variable to store timeout_msecs as well? Or do we
> want to make a userspace-visible change like this? I don't have a
> preference, I just want to make sure I understand.

Actually, given this, isn't the interface for sys_poll() incorrectly 
defined?
Shouldn't the timeout argument be an int, instead of a long?

And, if we make it an int, then can't we do the math correctly for all
possible values of the timeout?  The patch could look like:

Signed-off-by: Peter Staubach <staubach@xxxxxxxxxx>

--- linux-2.6.13/fs/select.c.org	2005-08-28 19:41:01.000000000 -0400
+++ linux-2.6.13/fs/select.c	2005-09-12 10:19:30.000000000 -0400
@@ -457,25 +457,34 @@ static int do_poll(unsigned int nfds,  s
 	return count;
 }
 
-asmlinkage long sys_poll(struct pollfd __user * ufds, unsigned int nfds, long timeout)
+asmlinkage long sys_poll(struct pollfd __user * ufds, unsigned int nfds, int timeout_msecs)
 {
 	struct poll_wqueues table;
  	int fdcount, err;
  	unsigned int i;
 	struct poll_list *head;
  	struct poll_list *walk;
+	long timeout;
+	int64_t lltimeout;
 
 	/* Do a sanity check on nfds ... */
 	if (nfds > current->files->max_fdset && nfds > OPEN_MAX)
 		return -EINVAL;
 
-	if (timeout) {
-		/* Careful about overflow in the intermediate values */
-		if ((unsigned long) timeout < MAX_SCHEDULE_TIMEOUT / HZ)
-			timeout = (unsigned long)(timeout*HZ+999)/1000+1;
-		else /* Negative or overflow */
+	if (timeout_msecs) {
+		if (timeout_msecs < 0)
 			timeout = MAX_SCHEDULE_TIMEOUT;
-	}
+		else {
+			lltimeout = (int64_t)timeout_msecs * HZ + 999;
+			do_div(lltimeout, 1000);
+			lltimeout++;
+			if (lltimeout > MAX_SCHEDULE_TIMEOUT)
+				timeout = MAX_SCHEDULE_TIMEOUT;
+			else
+				timeout = (long)lltimeout;
+		}
+	} else
+		timeout = 0;
 
 	poll_initwait(&table);
 
--- linux-2.6.13/include/linux/syscalls.h.org	2005-08-28 19:41:01.000000000 -0400
+++ linux-2.6.13/include/linux/syscalls.h	2005-09-12 10:22:25.000000000 -0400
@@ -420,7 +420,7 @@ asmlinkage long sys_socketpair(int, int,
 asmlinkage long sys_socketcall(int call, unsigned long __user *args);
 asmlinkage long sys_listen(int, int);
 asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,
-				long timeout);
+				int timeout_msecs);
 asmlinkage long sys_select(int n, fd_set __user *inp, fd_set __user *outp,
 			fd_set __user *exp, struct timeval __user *tvp);
 asmlinkage long sys_epoll_create(int size);