linux-kernel - [PATCH][RFC] unbreak generic futex_atomic_cmpxchg

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Date:	Wed, 3 Oct 2007 22:33:46 +0300
From:	Riku Voipio <riku.voipio@....fi>
To:	linux-kernel@...r.kernel.org
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	Lennert Buytenhek <buytenh@...tstofly.org>
Subject: [PATCH][RFC] unbreak generic futex_atomic_cmpxchg_inatomic() on UP

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What's the state of this patch? I can confirm tst-robust1
from glibc testsuite locks a armv5 machine hard. With this patch
applied, the test succeeds.

> The higher-end archs (x86, sparc64, ppc64, etc) provide fully-functional
> asm/futex.h implementations, but a number of archs (alpha, arm, arm26,
> avr32, blackfin, cris, h8300, m32r, m68k, mk68knommu, sh64, sparc, um,
> v850, and xtensa) use asm-generic/futex.h, which makes robust futexes
> horribly broken on them. There have also been reports recently that PI
> futexes are broken due to the generic futex_atomic_cmpxchg_inatomic()
> just being an -ENOSYS stub.

This an effective local DOS bug on the affected architectures, too.. 

> The patch below implements the generic futex_atomic_cmpxchg_inatomic() in
> terms of __copy_{from,to}_user_inatomic() and preempt_{disable,enable}().
> It obviously doesn't support SMP, but UP-only support should go a long
> way for users of the affected archs.
> 
> I'm using this patch now and it has allowed me to build and use glibc-2.4
> with NPTL on ARM (glibc-2.4-11.src.rpm from FC5 + ARM fixes).
> (Finally I can ditch LinuxThreads :->)
> 
> Comments?
> 
> /Mikael
> 
> --- linux-2.6.22/include/asm-generic/futex.h.~1~	2007-02-04 19:44:54.000000000 +0100
> +++ linux-2.6.22/include/asm-generic/futex.h	2007-08-01 19:03:43.000000000 +0200
> @@ -4,6 +4,7 @@
>  #ifdef __KERNEL__
>  
>  #include <linux/futex.h>
> +#include <linux/preempt.h>
>  #include <asm/errno.h>
>  #include <asm/uaccess.h>
>  
> @@ -52,7 +53,34 @@ futex_atomic_op_inuser (int encoded_op, 
>  static inline int
>  futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
> {
> +#ifdef CONFIG_SMP
>  	return -ENOSYS;
> +#else
> +	int curval, ret;
> +
> +	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
> +		return -EFAULT;
> +
> +	preempt_disable();
> +
> +	ret = -EFAULT;
> +	if (__copy_from_user_inatomic(&curval, uaddr, sizeof(int)))
> +		goto out;
> +
> +	ret = curval;
> +	if (curval != oldval)
> +		goto out;
> +
> +	ret = -EFAULT;
> +	if (__copy_to_user_inatomic(uaddr, &newval, sizeof(int)))
> +		goto out;
> +
> +	ret = newval;
> +
> + out:
> +	preempt_enable();
> +	return ret;
> +#endif
>  }
>  
>  #endif
> -

- -- 
"rm -rf" only sounds scary if you don't have backups
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