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Date: Fri, 15 Feb 2008 02:36:58 +0100 (CET)
From: Thomas Gleixner <tglx@...utronix.de>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: adrian.bunk@...ial.fi, mingo@...e.hu, linux-kernel@...r.kernel.org,
riku.voipio@...ial.fi, mikpe@...uu.se, buytenh@...tstofly.org,
rusty@...tcorp.com.au, stable@...nel.org
Subject: [PATCH 2/2] futex: runtime enable pi and robust functionality
Not all architectures implement futex_atomic_cmpxchg_inatomic(). The
default implementation returns -ENOSYS, which is currently not handled
inside of the futex guts.
Futex PI calls and robust list exits with a held futex result in an
endless loop in the futex code on architectures which have no support.
Fixing up every place where futex_atomic_cmpxchg_inatomic() is called
would add a fair amount of extra if/else constructs to the already
complex code. It is also not possible to disable the robust feature
before user space tries to register robust lists.
Compile time disabling is not a good idea either, as there are already
architectures with runtime detection of futex_atomic_cmpxchg_inatomic
support.
Detect the functionality at runtime instead by calling
cmpxchg_futex_value_locked() with a NULL pointer from the futex
initialization code. This is guaranteed to fail, but the call of
futex_atomic_cmpxchg_inatomic() happens with pagefaults disabled.
On architectures, which use the asm-generic implementation or have a
runtime CPU feature detection, a -ENOSYS return value disables the
PI/robust features.
On architectures with a working implementation the call returns
-EFAULT and the PI/robust features are enabled.
The relevant syscalls return -ENOSYS and the robust list exit code is
blocked, when the detection fails.
Fixes http://lkml.org/lkml/2008/2/11/149
Originally reported by: Lennart Buytenhek
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Ingo Molnar <mingo@...e.hu>
Cc: stable@...nel.org
---
include/linux/futex.h | 1 +
kernel/futex.c | 38 ++++++++++++++++++++++++++++++++++----
kernel/futex_compat.c | 9 +++++++++
3 files changed, 44 insertions(+), 4 deletions(-)
Index: linux-2.6/include/linux/futex.h
===================================================================
--- linux-2.6.orig/include/linux/futex.h
+++ linux-2.6/include/linux/futex.h
@@ -167,6 +167,7 @@ union futex_key {
#ifdef CONFIG_FUTEX
extern void exit_robust_list(struct task_struct *curr);
extern void exit_pi_state_list(struct task_struct *curr);
+extern int futex_cmpxchg_enabled;
#else
static inline void exit_robust_list(struct task_struct *curr)
{
Index: linux-2.6/kernel/futex.c
===================================================================
--- linux-2.6.orig/kernel/futex.c
+++ linux-2.6/kernel/futex.c
@@ -60,6 +60,8 @@
#include "rtmutex_common.h"
+int __read_mostly futex_cmpxchg_enabled;
+
#define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
/*
@@ -469,6 +471,8 @@ void exit_pi_state_list(struct task_stru
struct futex_hash_bucket *hb;
union futex_key key;
+ if (!futex_cmpxchg_enabled)
+ return;
/*
* We are a ZOMBIE and nobody can enqueue itself on
* pi_state_list anymore, but we have to be careful
@@ -1870,6 +1874,8 @@ asmlinkage long
sys_set_robust_list(struct robust_list_head __user *head,
size_t len)
{
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
/*
* The kernel knows only one size for now:
*/
@@ -1894,6 +1900,9 @@ sys_get_robust_list(int pid, struct robu
struct robust_list_head __user *head;
unsigned long ret;
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
+
if (!pid)
head = current->robust_list;
else {
@@ -1997,6 +2006,9 @@ void exit_robust_list(struct task_struct
unsigned long futex_offset;
int rc;
+ if (!futex_cmpxchg_enabled)
+ return;
+
/*
* Fetch the list head (which was registered earlier, via
* sys_set_robust_list()):
@@ -2051,7 +2063,7 @@ void exit_robust_list(struct task_struct
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
u32 __user *uaddr2, u32 val2, u32 val3)
{
- int ret;
+ int ret = -ENOSYS;
int cmd = op & FUTEX_CMD_MASK;
struct rw_semaphore *fshared = NULL;
@@ -2083,13 +2095,16 @@ long do_futex(u32 __user *uaddr, int op,
ret = futex_wake_op(uaddr, fshared, uaddr2, val, val2, val3);
break;
case FUTEX_LOCK_PI:
- ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
+ if (futex_cmpxchg_enabled)
+ ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
break;
case FUTEX_UNLOCK_PI:
- ret = futex_unlock_pi(uaddr, fshared);
+ if (futex_cmpxchg_enabled)
+ ret = futex_unlock_pi(uaddr, fshared);
break;
case FUTEX_TRYLOCK_PI:
- ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1);
+ if (futex_cmpxchg_enabled)
+ ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1);
break;
default:
ret = -ENOSYS;
@@ -2145,8 +2160,23 @@ static struct file_system_type futex_fs_
static int __init init(void)
{
+ u32 curval;
int i;
+ /*
+ * This will fail and we want it. Some arch implementations do
+ * runtime detection of the futex_atomic_cmpxchg_inatomic()
+ * functionality. We want to know that before we call in any
+ * of the complex code paths. Also we want to prevent
+ * registration of robust lists in that case. NULL is
+ * guaranteed to fault and we get -EFAULT on functional
+ * implementation, the non functional ones will return
+ * -ENOSYS.
+ */
+ curval = cmpxchg_futex_value_locked(NULL, 0, 0);
+ if (curval == -EFAULT)
+ futex_cmpxchg_enabled = 1;
+
for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
plist_head_init(&futex_queues[i].chain, &futex_queues[i].lock);
spin_lock_init(&futex_queues[i].lock);
Index: linux-2.6/kernel/futex_compat.c
===================================================================
--- linux-2.6.orig/kernel/futex_compat.c
+++ linux-2.6/kernel/futex_compat.c
@@ -54,6 +54,9 @@ void compat_exit_robust_list(struct task
compat_long_t futex_offset;
int rc;
+ if (!futex_cmpxchg_enabled)
+ return;
+
/*
* Fetch the list head (which was registered earlier, via
* sys_set_robust_list()):
@@ -115,6 +118,9 @@ asmlinkage long
compat_sys_set_robust_list(struct compat_robust_list_head __user *head,
compat_size_t len)
{
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
+
if (unlikely(len != sizeof(*head)))
return -EINVAL;
@@ -130,6 +136,9 @@ compat_sys_get_robust_list(int pid, comp
struct compat_robust_list_head __user *head;
unsigned long ret;
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
+
if (!pid)
head = current->compat_robust_list;
else {
--
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