Userspace helpers at static addresses on ARM [was: Re: [PATCH] fix the "unknown" case]

[Mathieu Desnoyers]

* Ulrich Weigand (Ulrich.Weigand at de.ibm.com) wrote:
> Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote on 06/15/2010
> 07:03:15 PM:
> 
> > I wonder starting with which Linux kernel version __kernel_dmb appeared.
> > Tying ourself directly to a Linux kernel ABI might complicate things.
> >
> > Is this ABI presented in a vDSO or userland have to go through a system
> call ?
> > Is there any way to probe for its availability ?
> 
> This looks sort-of like a vDSO, except without the DSO part :-)
> 
> The kernel simply makes the code available at a fixed address that is
> directly callable by user space.  See the comments in
> linux/arch/arm/kernel/entry-armv.S:
> 

Hrm, statically addressed shared objects. The security guys should be freaking
out here. This can sadly make stack overflow exploitation much, much, easier
because of lack of randomization of addresses where the code is located. :-/

About the original topic of our discussion:
Thanks for the explanation below. I think making urcu test for the kernel
feature at library load seems like the best portable solution so far. We can
directly use the specific memory barriers when armv7+ is specified, and check
at runtime if the kernel feature is there for "generic" arm build. For generic
ARM build where we discover that the kernel lacks the proper features, we could
rely on Paul's double-fake-mutex scheme (assuming we audit glibc pthreads to
ensure the proper memory barriers are there). If we find out that even pthreads
mutexes got the barriers wrong there, then we should refuse to load the library
altogether.

Thanks,

Mathieu

> /*
>  * User helpers.
>  *
>  * These are segment of kernel provided user code reachable from user space
>  * at a fixed address in kernel memory.  This is used to provide user space
>  * with some operations which require kernel help because of unimplemented
>  * native feature and/or instructions in many ARM CPUs. The idea is for
>  * this code to be executed directly in user mode for best efficiency but
>  * which is too intimate with the kernel counter part to be left to user
>  * libraries.  In fact this code might even differ from one CPU to another
>  * depending on the available  instruction set and restrictions like on
>  * SMP systems.  In other words, the kernel reserves the right to change
>  * this code as needed without warning. Only the entry points and their
>  * results are guaranteed to be stable.
>  *
>  * Each segment is 32-byte aligned and will be moved to the top of the high
>  * vector page.  New segments (if ever needed) must be added in front of
>  * existing ones.  This mechanism should be used only for things that are
>  * really small and justified, and not be abused freely.
>  *
>  * User space is expected to implement those things inline when optimizing
>  * for a processor that has the necessary native support, but only if such
>  * resulting binaries are already to be incompatible with earlier ARM
>  * processors due to the use of unsupported instructions other than what
>  * is provided here.  In other words don't make binaries unable to run on
>  * earlier processors just for the sake of not using these kernel helpers
>  * if your compiled code is not going to use the new instructions for other
>  * purpose.
>  */
> 
> 
> /*
>  * Reference prototype:
>  *
>  *           void __kernel_memory_barrier(void)
>  *
>  * Input:
>  *
>  *           lr = return address
>  *
>  * Output:
>  *
>  *           none
>  *
>  * Clobbered:
>  *
>  *           none
>  *
>  * Definition and user space usage example:
>  *
>  *           typedef void (__kernel_dmb_t)(void);
>  *           #define __kernel_dmb (*(__kernel_dmb_t *)0xffff0fa0)
>  *
>  * Apply any needed memory barrier to preserve consistency with data
> modified
>  * manually and __kuser_cmpxchg usage.
>  *
>  * This could be used as follows:
>  *
>  * #define __kernel_dmb() \
>  *         asm volatile ( "mov r0, #0xffff0fff; mov lr, pc; sub pc, r0,
> #95" \
>  *		         : : : "r0", "lr","cc" )
>  */
> 
> 
> As far as I can see, the only provision to check whether a feature is
> available
> is this one:
> 
> /*
>  * Reference declaration:
>  *
>  *           extern unsigned int __kernel_helper_version;
>  *
>  * Definition and user space usage example:
>  *
>  *           #define __kernel_helper_version (*(unsigned int *)0xffff0ffc)
>  *
>  * User space may read this to determine the curent number of helpers
>  * available.
>  */
> 
> However, note that libgcc code does not perform this check, it simply
> assumes
> the above routine to be present.
> 
> The __kernel_dmb (which is the most recently added helper available in
> current
> mainline) seems to have been available since kernel 2.6.15, so presumably
> code
> using any of the GCC sync primitives would simply fail on any older kernel,
> unless I'm missing something here ...
> 
> 
> Mit freundlichen Gruessen / Best Regards
> 
> Ulrich Weigand
> 
> --
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com