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

Peter Staubach
2005-09-12T14:38:12Z

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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);