Re: [RFC] sys_setrlimit() in 2.6.16

[Andrew Morton <akpm@osdl.org>]

Cliff Wickman <cpw@sgi.com> wrote:
>
> A test suite uncovered a possible bug in setrlimit(2), in 2.6.16-rc3.
> 
> A code that does
>         mylimits.rlim_cur = 0;
>         setrlimit(RLIMIT_CPU, &mylimits);
> does not set a cpu time limit.
> 
> No signal is sent to this code when its "limit" of 0 seconds expires.
> Whereas, under the 2.6.5 kernel (SuSE SLESS9) a signal was sent.
> 
> I don't see any obvious difference in sys_setrlimit() or
> set_process_cpu_timer() between 2.6.5 and 2.6.16.
> 
> Is this a correction, or a bug?
> 
> Is a cpu time limit of 0 supposed to limit a task to 0 seconds, or
> leave it unlimited?
> 

This has to be considered a bug.  The spec certainly implies that a limit
of zero should be honoured and, probably more importantly, that's how it
works in 2.4.

Problem is, the code in there all assumes that an it_prof_expires of zero
means "it was never set", and changing that (add a yes-it-has flag?) would
be less than trivial.

So I think the path of least resistance here is to just convert the
caller's zero seconds into one second.  That in fact gives the same
behaviour as 2.4: you get whacked after one second or more CPU time.

(This is not a final patch - that revolting expression in sys_setrlimit()
needs help first).


diff -puN kernel/sys.c~a kernel/sys.c
--- devel/kernel/sys.c~a	2006-02-16 00:42:49.000000000 -0800
+++ devel-akpm/kernel/sys.c	2006-02-16 00:45:10.000000000 -0800
@@ -1657,7 +1657,19 @@ asmlinkage long sys_setrlimit(unsigned i
 	    (cputime_eq(current->signal->it_prof_expires, cputime_zero) ||
 	     new_rlim.rlim_cur <= cputime_to_secs(
 		     current->signal->it_prof_expires))) {
-		cputime_t cputime = secs_to_cputime(new_rlim.rlim_cur);
+		unsigned long rlim_cur = new_rlim.rlim_cur;
+		cputime_t cputime;
+
+		if (rlim_cur == 0) {
+			/*
+			 * The caller is asking for an immediate RLIMIT_CPU
+			 * expiry.  But we use the zero value to mean "it was
+			 * never set".  So let's cheat and make it one second
+			 * instead
+			 */
+			rlim_cur = 1;
+		}
+		cputime = secs_to_cputime(rlim_cur);
 		read_lock(&tasklist_lock);
 		spin_lock_irq(&current->sighand->siglock);
 		set_process_cpu_timer(current, CPUCLOCK_PROF,
_