path: root/kernel/futex_compat.c
diff options
authorThomas Gleixner <tglx@linutronix.de>2008-02-23 15:23:57 -0800
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-02-23 17:12:15 -0800
commita0c1e9073ef7428a14309cba010633a6cd6719ea (patch)
tree05ce792ddcde92e73d1bae4c8e20f607a2e7db40 /kernel/futex_compat.c
parent3e4ab747efa8e78562ec6782b08bbf21a00aba1b (diff)
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>
Diffstat (limited to 'kernel/futex_compat.c')
1 files changed, 9 insertions, 0 deletions
diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 7d5e4b016f3..ff90f049f8f 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -54,6 +54,9 @@ void compat_exit_robust_list(struct task_struct *curr)
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, compat_uptr_t __user *head_ptr,
struct compat_robust_list_head __user *head;
unsigned long ret;
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
if (!pid)
head = current->compat_robust_list;
else {