linux-kernel - [rfc] "improve" preempt debugging

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 30 Sep 2008 20:50:27 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Molnar, Ingo" <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: [rfc] "improve" preempt debugging

Hi, don't know if you really like this patch or not, but it helped me out with
a problem I recently had....

Basically, when the kernel lock is held, then preempt_count underflow does not
get detected until it is released which may be a long time (and arbitrarily,
eg at different points it may be rescheduled). If the bkl is released at
schedule, the resulting output is actually fairly cryptic...

With any other lock that elevates preempt_count, it is illegal to schedule
under it (which would get found pretty quickly). bkl allows scheduling with
preempt_count elevated, which makes underflows hard to debug.

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c       2008-09-30 11:32:56.000000000 +1000
+++ linux-2.6/kernel/sched.c    2008-09-30 11:38:18.000000000 +1000
@@ -4305,7 +4305,7 @@ void __kprobes sub_preempt_count(int val
        /*
         * Underflow?
         */
-       if (DEBUG_LOCKS_WARN_ON(val > preempt_count()))
+       if (DEBUG_LOCKS_WARN_ON(val > preempt_count() - (!!kernel_locked())))
                return;
        /*
         * Is the spinlock portion underflowing?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/