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

Cyrill Gorcunov
2012-02-16T16:49:10Z

On Thu, Feb 16, 2012 at 07:13:40PM +0400, Cyrill Gorcunov wrote:
...
>
> > You could simply do
> > int mutex_double_lock_killable(struct mutex *m1, struct mutex *m2)
> > {
> > int err;
> >
> > if (m2 > m1)
> > swap(m1, m2);
> >
> > err = mutex_lock_killable(m1);
> >
> > if (!err && likely(m1 != m2)) {
> > err = mutex_lock_killable_nested(m2);
> > if (err)
> > mutex_unlock(m1);
> > }
> >
> > return err;
> > }
> >
> > but I won't insist.
>
> Initially I wanted kcmp would be brining minimum impact
> and if mutex is already taken by someone, we would not sleep
> but return immediately with -EBUSY and it would be up to caller
> to deside if to try again or make some delay first. I simply
> not sure what is better here.
>
This one should do the trick.

Cyrill
---
From: Cyrill Gorcunov <gorcunov@openvz.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@redhat.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.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 +58,31 @@
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 (m2 > m1)
+ swap(m1, m2);
+
+ 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 (m2 > m1)
+ swap(m1, m2);

- if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
- mutex_unlock(&task->signal->cred_guard_mutex);
- return -EPERM;
+ 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 +109,15 @@
/*
* 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 +161,9 @@
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);
}