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SubjectRe: [PATCH] Prevent max_threads from exceeding PID_MAX
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On Thu, 2002-03-07 at 14:45, Patrick O'Rourke wrote:
> It is possible on large memory systems that the default process limits
> can exceed PID_MAX. This will allow a non-root user to consume all pids
> resulting in the kernel to basically hang in get_pid().
>

> + /* don't let threads go beyond PID_MAX */
> + if (max_threads > PID_MAX) {
> + max_threads = PID_MAX;
> + }
> +
The problem with this approach is that it doesn't take into account
pgrp, tgids, etc... I submitted the following patch a couple of weeks
ago that fixes the problem a better way.

Thanks,
Paul Larson

diff -Naur linux-2.4.18-rc2/kernel/fork.c linux-getpid/kernel/fork.c
--- linux-2.4.18-rc2/kernel/fork.c Wed Feb 20 09:54:39 2002
+++ linux-getpid/kernel/fork.c Fri Feb 22 15:52:52 2002
@@ -20,6 +20,7 @@
#include <linux/vmalloc.h>
#include <linux/completion.h>
#include <linux/personality.h>
+#include <linux/compiler.h>

#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@@ -85,12 +86,13 @@
{
static int next_safe = PID_MAX;
struct task_struct *p;
- int pid;
+ int pid, beginpid;

if (flags & CLONE_PID)
return current->pid;

spin_lock(&lastpid_lock);
+ beginpid = last_pid;
if((++last_pid) & 0xffff8000) {
last_pid = 300; /* Skip daemons etc. */
goto inside;
@@ -110,12 +112,16 @@
last_pid = 300;
next_safe = PID_MAX;
}
+ if(unlikely(last_pid == beginpid))
+ goto nomorepids;
goto repeat;
}
if(p->pid > last_pid && next_safe > p->pid)
next_safe = p->pid;
if(p->pgrp > last_pid && next_safe > p->pgrp)
next_safe = p->pgrp;
+ if(p->tgid > last_pid && next_safe > p->tgid)
+ next_safe = p->tgid;
if(p->session > last_pid && next_safe > p->session)
next_safe = p->session;
}
@@ -125,6 +131,11 @@
spin_unlock(&lastpid_lock);

return pid;
+
+nomorepids:
+ read_unlock(&tasklist_lock);
+ spin_unlock(&lastpid_lock);
+ return 0;
}

static inline int dup_mmap(struct mm_struct * mm)
@@ -620,6 +631,8 @@

copy_flags(clone_flags, p);
p->pid = get_pid(clone_flags);
+ if (p->pid == 0 && current->pid != 0)
+ goto bad_fork_cleanup;

p->run_list.next = NULL;
p->run_list.prev = NULL;
-
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