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@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Cc: Riku Voipio <riku.voipio@movial.fi>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 34 insertions, 4 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index c21f667c63f6..06968cd79200 100644
--- a/kernel/futex.c
+++ b/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_struct *curr)
         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 robust_list_head __user * __user *head_ptr,
         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 *curr)
         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 *curr)
 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, u32 val, ktime_t *timeout,
                 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_type = {
 
 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);