linux-kernel - Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

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Date:	Thu, 16 Feb 2012 23:29:52 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Vasiliy Kulikov <segoon@...nwall.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Alexey Dobriyan <adobriyan@...il.com>, Valdis.Kletnieks@...edu,
	Michal Marek <mmarek@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

On Thu, Feb 16, 2012 at 08:03:21PM +0100, Oleg Nesterov wrote:
> On 02/16, Cyrill Gorcunov wrote:
> >
> > On Thu, Feb 16, 2012 at 06:40:47PM +0100, Oleg Nesterov wrote:
> > > On 02/16, Cyrill Gorcunov wrote:
> > > >
> > > > -static void access_unlock(struct task_struct *task)
> > > > +static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
> > > >  {
> > > > -	mutex_unlock(&task->signal->cred_guard_mutex);
> > > > +	if (m2 > m1)
> > > > +		swap(m1, m2);
> > >
> > > Well, the order doesn't matter in case of _unlock, you can remove
> > > this part. Not that it really hurts though, I won't argue.
> >
> > It drops some instructions so I think it worth removing
> 
> Yes.
> 

Final one ;) I agreed on every line of your comment, thanks a lot Oleg!
---
From: Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: syscalls, x86: Make __NR_kcmp to work with equivalent pids

In case if pid1 is equal to pid2 the kcmp will return -EBUSY,
which makes no sence. Make it able to work with equivalent pids.
Selftest is extended as well.

Repored-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
---
diff -u linux-2.6.git/kernel/kcmp.c linux-2.6.git/kernel/kcmp.c
--- linux-2.6.git/kernel/kcmp.c
+++ linux-2.6.git/kernel/kcmp.c
@@ -58,22 +58,28 @@
 	return file;
 }
 
-static void access_unlock(struct task_struct *task)
+static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
 {
-	mutex_unlock(&task->signal->cred_guard_mutex);
+	if (likely(m2 != m1))
+		mutex_unlock(m2);
+	mutex_unlock(m1);
 }
 
-static int access_trylock(struct task_struct *task)
+static int kcmp_lock(struct mutex *m1, struct mutex *m2)
 {
-	if (!mutex_trylock(&task->signal->cred_guard_mutex))
-		return -EBUSY;
+	int err;
 
-	if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
-		mutex_unlock(&task->signal->cred_guard_mutex);
-		return -EPERM;
+	if (m2 > m1)
+		swap(m1, m2);
+
+	err = mutex_lock_killable(m1);
+	if (!err && likely(m1 != m2)) {
+		err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
+		if (err)
+			mutex_unlock(m1);
 	}
 
-	return 0;
+	return err;
 }
 
 SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
@@ -100,12 +106,15 @@
 	/*
 	 * One should have enough rights to inspect task details.
 	 */
-	ret = access_trylock(task1);
+	ret = kcmp_lock(&task1->signal->cred_guard_mutex,
+			&task2->signal->cred_guard_mutex);
 	if (ret)
 		goto err;
-	ret = access_trylock(task2);
-	if (ret)
+	if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
+	    !ptrace_may_access(task2, PTRACE_MODE_READ)) {
+		ret = -EPERM;
 		goto err_unlock;
+	}
 
 	switch (type) {
 	case KCMP_FILE: {
@@ -149,9 +158,9 @@
 		break;
 	}
 
-	access_unlock(task2);
 err_unlock:
-	access_unlock(task1);
+	kcmp_unlock(&task1->signal->cred_guard_mutex,
+		    &task2->signal->cred_guard_mutex);
 err:
 	put_task_struct(task1);
 	put_task_struct(task2);
diff -u linux-2.6.git/tools/testing/selftests/kcmp/kcmp_test.c linux-2.6.git/tools/testing/selftests/kcmp/kcmp_test.c
--- linux-2.6.git/tools/testing/selftests/kcmp/kcmp_test.c
+++ linux-2.6.git/tools/testing/selftests/kcmp/kcmp_test.c
@@ -74,6 +74,15 @@
 			ret = -1;
 		} else
 			printf("PASS: 0 returned as expected\n");
+
+		/* Compare with self */
+		ret = sys_kcmp(pid1, pid1, KCMP_VM, 0, 0);
+		if (ret) {
+			printf("FAIL: 0 expected but %li returned\n", ret);
+			ret = -1;
+		} else
+			printf("PASS: 0 returned as expected\n");
+
 		exit(ret);
 	}
 
--
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