Gustavo Chain wrote:
> Al Boldi <[email protected]> escribió:
> > Kyle Moffett wrote:
> > > On Oct 12, 2007, at 01:37:23, Al Boldi wrote:
> > > > You have a point, and resource-controllers can probably control
> > > > DoS a lot better, but the they also incur more overhead. Think
> > > > of this "lockout prevention" patch as a near zero overhead safety
> > > > valve.
> > >
> > > But why do you need to add "lockout prevention" if it already
> > > exists?
> > I said this before, but I'll say it again: it's about overhead!
> > > With CFS' extremely efficient per-user-scheduling (hopefully
> > > soon to be the default) there are only two forms of lockout by non-
> > > root processes: (1) Running out of PIDs in the box's PID-space
> > > (think tens or hundreds of thousands of processes), or (2) Swap-
> > > storming the box to death. To put it bluntly trying to reserve free
> > > PID slots is attacking the wrong end of the problem and your so
> > > called "lockout prevention" could very easily ensure that 10 PIDs
> > > are available even if the user has swapstormed the box with the
> > > PIDs he does have.
> > I think you are reading this wrong. It's not about reserving PIDs,
> > it's about exceeding the max-threads limit. This limit is global and
> > affects every user including root, which is good, as this allows the
> > sysadmin to fence the system into a controllable state. So once the
> > system reaches the fence, sysadmin-intervention allows root to exceed
> > the fence.
> > Again, this is much nicer with real resource-controllers, but again
> > it's also more overhead.
> Just an _if()_ ?
> may be enable it as an option in kernel config ?
Here is the patch again:
[PATCH 1/1] threads_max: Simple lockout prevention patch
Simple attempt to provide a backdoor in a process lockout situation.
echo $$ > /proc/sys/kernel/su-pid allows pid to exceed the threads_max limit.
Note that this patch incurs zero runtime-overhead.
Signed-off-by: Al Boldi <[email protected]>
(patch against 2.6.14)
--- kernel/fork.c.orig 2005-11-14 20:55:33.000000000 +0300
+++ kernel/fork.c 2005-11-14 20:58:25.000000000 +0300
@@ -57,6 +57,7 @@
int nr_threads; /* The idle threads do not count.. */
int max_threads; /* tunable limit on nr_threads */
+int su_pid; /* BackDoor pid to exceed limit on nr_threads */
DEFINE_PER_CPU(unsigned long, process_counts) = 0;
@@ -926,6 +927,7 @@
* to stop root fork bombs.
if (nr_threads >= max_threads)
+ if (p->pid != su_pid)
--- kernel/sysctl.c.orig 2005-11-14 20:58:45.000000000 +0300
+++ kernel/sysctl.c 2005-11-14 21:01:20.000000000 +0300
@@ -57,6 +57,7 @@
extern int sysctl_overcommit_memory;
extern int sysctl_overcommit_ratio;
extern int max_threads;
+extern int su_pid;
extern int sysrq_enabled;
extern int core_uses_pid;
extern int suid_dumpable;
@@ -509,6 +510,14 @@
.proc_handler = &proc_dointvec,
+ .ctl_name = KERN_SU_PID,
+ .procname = "su-pid",
+ .data = &su_pid,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
.ctl_name = KERN_RANDOM,
.procname = "random",
.mode = 0555,
--- include/linux/sysctl.h.orig 2005-11-14 20:54:55.000000000 +0300
+++ include/linux/sysctl.h 2005-11-14 20:55:15.000000000 +0300
@@ -146,6 +146,7 @@
KERN_RANDOMIZE=68, /* int: randomize virtual address space */
KERN_SETUID_DUMPABLE=69, /* int: behaviour of dumps for setuid core
KERN_SPIN_RETRY=70, /* int: number of spinlock retries */
+ KERN_SU_PID=71, /* int: BackDoor pid to exceed Maximum
+ /* nr of threads in the system */
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