Re: [PATCH] Re: itimer oddness in 2.6.12
Andrew Morton wrote: George Anzinger wrote: + while (time_before_eq(p->signal->real_timer.expires, jiffies)) + p->signal->real_timer.expires += inc; It gives me the creeps when I see timer code doing this, and it seems to be done relatively frequently. Surely it can be calculated arithmetically? If not, are you really sure that it is not exploitable by malicious code? Hm.. the system only falls into a loop here if the system is loaded to the point where we are a jiffie or more late. The prior code just did the "+=" and called add_timer, possibly with a time in the past. I suspect that way of doing this would never catch up if the user asked for a one jiffie repeat time. Also, this is faster than the div, mpy if you are not late (or even if you are several jiffies late). A possible alternative might be: p->signal->real_timer.expires += inc; if (time_before_eq(p->signal->real_timer.expires, jiffies)) p->signal->real_timer.expires += ((jiffies - p->signal->real_timer.expires + inc -1) / inc) * inc; Both a div and a mpy in there. I really think the "while" is ok, but if you prefer... The last time you questioned this sort of thing was in the code to correct an absolute timer. In that case we were adjusting after a clock set and, yes, it was possibly exploitable (assuming you could set the clock). Here we don't have that possibility, i.e. we only get into the loop if the system is late. - -- George Anzinger george@mvista.com HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH] Re: itimer oddness in 2.6.12
George Anzinger wrote: > > + while (time_before_eq(p->signal->real_timer.expires, jiffies)) > + p->signal->real_timer.expires += inc; It gives me the creeps when I see timer code doing this, and it seems to be done relatively frequently. Surely it can be calculated arithmetically? If not, are you really sure that it is not exploitable by malicious code? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH] Re: itimer oddness in 2.6.12
George Anzinger wrote: Tom Marshall wrote: On Fri, Jul 22, 2005 at 08:21:25PM +0100, Paulo Marques wrote: Tom Marshall wrote: The patch to fix "setitimer timer expires too early" is causing issues for the Helix server. We have a timer processs that updates the server's timestamp on an itimer and it expects the signal to be delivered at roughly the interval retrieved from getitimer. This is very consistent on every platform, including Linux up to 2.6.11, but breaks on 2.6.12. On 2.6.12, setting the itimer to 10ms and retrieving the actual interval from getitimer reports 10.998ms, but the timer interrupts are consistently delivered at roughly 11.998ms. Unfortunately, this is not so clear cut as it seems :( Oops! That patch is wrong. The +1 should be applied to the initial interval _only_. We KNOW when the repeating intervals start (i.e. at the jiffie edge) and don't need to adjust them. The patch, however, incorrectly, rolls them all into one. The attach patch should fix the problem. Warnning, it compiles and boots, but I have not tested it. Can this get into 2.6.13? Or stable if it's too late? This would appear to be a fix to a visible problem. -- -bill davidsen ([EMAIL PROTECTED]) "The secret to procrastination is to put things off until the last possible moment - but no longer" -me - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH] Re: itimer oddness in 2.6.12
George Anzinger wrote: Tom Marshall wrote: On Fri, Jul 22, 2005 at 08:21:25PM +0100, Paulo Marques wrote: [...] Unfortunately, this is not so clear cut as it seems :( Oops! That patch is wrong. The +1 should be applied to the initial interval _only_. We KNOW when the repeating intervals start (i.e. at the jiffie edge) and don't need to adjust them. The patch, however, incorrectly, rolls them all into one. The attach patch should fix the problem. Warnning, it compiles and boots, but I have not tested it. Yes, this seems to be the Right Thing :) Acked-by: Paulo Marques <[EMAIL PROTECTED]> -- Paulo Marques - www.grupopie.com It is a mistake to think you can solve any major problems just with potatoes. Douglas Adams - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
[PATCH] Re: itimer oddness in 2.6.12
Tom Marshall wrote:
On Fri, Jul 22, 2005 at 08:21:25PM +0100, Paulo Marques wrote:
Tom Marshall wrote:
The patch to fix "setitimer timer expires too early" is causing issues for
the Helix server. We have a timer processs that updates the server's
timestamp on an itimer and it expects the signal to be delivered at roughly
the interval retrieved from getitimer. This is very consistent on every
platform, including Linux up to 2.6.11, but breaks on 2.6.12. On 2.6.12,
setting the itimer to 10ms and retrieving the actual interval from
getitimer
reports 10.998ms, but the timer interrupts are consistently delivered at
roughly 11.998ms.
Unfortunately, this is not so clear cut as it seems :(
Oops! That patch is wrong. The +1 should be applied to the initial
interval _only_. We KNOW when the repeating intervals start (i.e. at
the jiffie edge) and don't need to adjust them. The patch, however,
incorrectly, rolls them all into one. The attach patch should fix the
problem. Warnning, it compiles and boots, but I have not tested it.
George
--
Yes, I am sure that it is not a simple problem. I am not a kernel developer
but I imagine that issues such as NTP adjustments would complicate this
issue. I must also admit that I am not intimately familiar with the POSIX
spec regarding itimers.
Our current code does a setitimer followed by getitimer, then uses the
actual interval retrieved by getitimer to set a global timer delta. On each
timer signal, it updates the notion of the current time by the timer delta.
As mentioned, this works on every other platform (Solaris, BSD, HPUX, AIX,
DGUX, IRIX, Tru64, and Linux up to 2.6.11) but breaks on 2.6.12.
This is not an insurmountable problem for userspace. It can be easily
solved by using gettimeofday in the timer interrupt instead of adding the
delta to the current time blindly. No big deal. I just wanted to point
this issue out and ensure that (1) it was a known issue, and (2) it is the
direction that the Linux kernel intends to take. If so, no big deal and we
can modify the timer code to take that into account.
--
George Anzinger george@mvista.com
HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/
Source: MontaVista Software, Inc.
Type: Defect Fix
Disposition:
Description:
This changes setitimer as follows:
1. The repeating timer is figured using the requested time
(not +1 as we know where we are in the jiffie).
2. The tests for interval too large are left to the time_val to jiffie code.
Signed-off-by: George Anzinger
itimer.c | 37 -
1 files changed, 16 insertions(+), 21 deletions(-)
Index: linux-2.6.13-rc/kernel/itimer.c
===
--- linux-2.6.13-rc.orig/kernel/itimer.c
+++ linux-2.6.13-rc/kernel/itimer.c
@@ -112,28 +112,11 @@ asmlinkage long sys_getitimer(int which,
return error;
}
-/*
- * Called with P->sighand->siglock held and P->signal->real_timer inactive.
- * If interval is nonzero, arm the timer for interval ticks from now.
- */
-static inline void it_real_arm(struct task_struct *p, unsigned long interval)
-{
- p->signal->it_real_value = interval; /* XXX unnecessary field?? */
- if (interval == 0)
- return;
- if (interval > (unsigned long) LONG_MAX)
- interval = LONG_MAX;
- /* the "+ 1" below makes sure that the timer doesn't go off before
-* the interval requested. This could happen if
-* time requested % (usecs per jiffy) is more than the usecs left
-* in the current jiffy */
- p->signal->real_timer.expires = jiffies + interval + 1;
- add_timer(&p->signal->real_timer);
-}
void it_real_fn(unsigned long __data)
{
struct task_struct * p = (struct task_struct *) __data;
+ unsigned long inc = p->signal->it_real_incr;
send_group_sig_info(SIGALRM, SEND_SIG_PRIV, p);
@@ -141,14 +124,23 @@ void it_real_fn(unsigned long __data)
* Now restart the timer if necessary. We don't need any locking
* here because do_setitimer makes sure we have finished running
* before it touches anything.
+* Note, we KNOW we are (or should be) at a jiffie edge here so
+* we don't need the +1 stuff. Also, we want to use the prior
+* expire value so as to not "slip" a jiffie if we are late.
+* Deal with requesting a time prior to "now" here rather than
+* in add_timer.
*/
- it_real_arm(p, p->signal->it_real_incr);
+ if (!inc)
+ return;
+ while (time_before_eq(p->signal->real_timer.expires, jiffies))
+ p->signal->real_timer.expires += inc;
+ add_timer(&p->signal->real_timer);
}
int do_setitimer(int which, struct itimerval *value, struct itimerval *ovalue)
{
struct task_struct *tsk = current;
- unsigned long val, interval;
+ unsigned long
