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

[Peter Staubach <staubach@redhat.com>]

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Nishanth Aravamudan wrote:

>On 09.09.2005 [19:36:21 -0700], Andrew Morton wrote:
>  
>
>>Nishanth Aravamudan <nacc@us.ibm.com> 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@redhat.com>


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--- 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);