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Subject[PATCH] vt: kill tty->count usage
The commit e0426e6a09954d205da2d674a3d368d2715e3afd fixes a real race but
still isn't enough to prevent these:

kobject_add_internal failed for vcs7 with -EEXIST, don't try to register
things with the same name in the same direc.
Pid: 2967, comm: vcs_stress Not tainted 2.6.25 #10
[<c04e852e>] kobject_add_internal+0x137/0x149
[<c04e85d4>] kobject_add_varg+0x35/0x41
[<c04e8645>] kobject_add+0x43/0x49
[<c055e54f>] device_add+0x91/0x4b4
[<c055e984>] device_register+0x12/0x15
[<c055e9f6>] device_create+0x6f/0x95
[<c053e69b>] vcs_make_sysfs+0x23/0x4f
[<c0543aff>] con_open+0x74/0x82
[<c0538b64>] tty_open+0x188/0x287
[<c047b1c8>] chrdev_open+0x119/0x135
[<c04775d4>] __dentry_open+0xcf/0x185
[<c0477711>] nameidata_to_filp+0x1f/0x33
[<c047b0af>] ? chrdev_open+0x0/0x135
[<c0477753>] do_filp_open+0x2e/0x35
[<c0627da6>] ? _spin_unlock+0x1d/0x20
[<c04774ef>] ? get_unused_fd_flags+0xc9/0xd3
[<c047779a>] do_sys_open+0x40/0xb5
[<c0477851>] sys_open+0x1e/0x26
[<c0404962>] syscall_call+0x7/0xb
=======================

and the reason for that is a race in release_dev() that can call driver's close
two or more at the same time. If these are the last two references, both calls
will be made before tty->count is decremented. And most tty drivers check
tty->count to identify the last close so the resources can be released. So
the close() method and the tty->count should happen without concurrency or
each driver can keep the number of the references itself. Holding tty_mutex
before calling driver's close() and updating tty->count is not possible
because some drivers sleep waiting until the tx buffer is empty.

This patch was tested under normal usage and using the stress application I
wrote that triggers the problem more easily.

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>

---

drivers/char/vt.c | 82 +++++++++++++++++++++++++++++------------
include/linux/console_struct.h | 2 +
2 files changed, 60 insertions(+), 24 deletions(-)

--- a/drivers/char/vt.c 2008-08-07 16:06:55.000000000 -0400
+++ b/drivers/char/vt.c 2008-08-08 17:20:19.000000000 -0400
@@ -2730,15 +2730,24 @@ static int con_open(struct tty_struct *t
{
unsigned int currcons = tty->index;
int ret = 0;
+ struct vc_data *vc;

acquire_console_sem();
if (tty->driver_data == NULL) {
ret = vc_allocate(currcons);
if (ret == 0) {
- struct vc_data *vc = vc_cons[currcons].d;
+ vc = vc_cons[currcons].d;
+
+ if (vc->vc_tty != NULL) {
+ /* we're still releasing this console entry */
+ release_console_sem();
+ return -EBUSY;
+ }
+
tty->driver_data = vc;
vc->vc_tty = tty;

+ kref_init(&vc->kref);
if (!tty->winsize.ws_row && !tty->winsize.ws_col) {
tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;
tty->winsize.ws_col = vc_cons[currcons].d->vc_cols;
@@ -2751,39 +2760,63 @@ static int con_open(struct tty_struct *t
release_console_sem();
return ret;
}
+ } else {
+ vc = tty->driver_data;
+ kref_get(&vc->kref);
}
release_console_sem();
return ret;
}

-/*
- * We take tty_mutex in here to prevent another thread from coming in via init_dev
- * and taking a ref against the tty while we're in the process of forgetting
- * about it and cleaning things up.
- *
- * This is because vcs_remove_sysfs() can sleep and will drop the BKL.
- */
-static void con_close(struct tty_struct *tty, struct file *filp)
+static void con_release(struct kref *kref)
{
- mutex_lock(&tty_mutex);
+ struct vc_data *vc = container_of(kref, struct vc_data, kref);
+ struct tty_struct *tty = vc->vc_tty;
+
+ tty->driver_data = NULL;
+
+ /* we must release the semaphore here: vcs_remove_sysfs() may sleep */
+ release_console_sem();
+ vcs_remove_sysfs(tty);
acquire_console_sem();
- if (tty && tty->count == 1) {
- struct vc_data *vc = tty->driver_data;

- if (vc)
- vc->vc_tty = NULL;
- tty->driver_data = NULL;
- vcs_remove_sysfs(tty);
- release_console_sem();
- mutex_unlock(&tty_mutex);
- /*
- * tty_mutex is released, but we still hold BKL, so there is
- * still exclusion against init_dev()
- */
+ /* now this slot is officially free */
+ vc->vc_tty = NULL;
+}
+
+static void con_close(struct tty_struct *tty, struct file *filp)
+{
+ struct vc_data *vc = tty->driver_data;
+
+ /*
+ * if this tty was hung up, all the resources have been freed already
+ */
+ if (tty_hung_up_p(filp))
return;
- }
+
+ acquire_console_sem();
+ /*
+ * tty->driver_data can be NULL if con_open() fails because tty layer
+ * will call us even if the first open wasn't successful.
+ */
+ if (vc != NULL)
+ kref_put(&vc->kref, con_release);
+ release_console_sem();
+}
+
+/*
+ * hangup was called: release all resources. all the currently open files will
+ * have the file_operations changed to hung_up_tty_fops. this means that the
+ * only choice the application has is close and open again.
+ */
+static void con_hangup(struct tty_struct *tty)
+{
+ struct vc_data *vc = tty->driver_data;
+
+ acquire_console_sem();
+ BUG_ON(vc == NULL);
+ con_release(&vc->kref);
release_console_sem();
- mutex_unlock(&tty_mutex);
}

static int default_italic_color = 2; // green (ASCII)
@@ -2907,6 +2940,7 @@ static const struct tty_operations con_o
.start = con_start,
.throttle = con_throttle,
.unthrottle = con_unthrottle,
+ .hangup = con_hangup,
};

int __init vty_init(void)
--- a/include/linux/console_struct.h 2008-05-23 15:53:23.000000000 -0400
+++ b/include/linux/console_struct.h 2008-08-08 11:55:49.000000000 -0400
@@ -15,6 +15,7 @@
#include <linux/wait.h>
#include <linux/vt.h>
#include <linux/workqueue.h>
+#include <linux/kref.h>

struct vt_struct;

@@ -108,6 +109,7 @@ struct vc_data {
unsigned long vc_uni_pagedir;
unsigned long *vc_uni_pagedir_loc; /* [!] Location of uni_pagedir variable for this console */
/* additional information is in vt_kern.h */
+ struct kref kref;
};

struct vc {

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