[LTP] ltp_clone

[LTP] ltp_clone

[Will Deacon]

Hello,

The definition of ltp_clone looks like this:


int
ltp_clone(unsigned long clone_flags, int (*fn)(void *arg), void *arg,
		size_t stack_size, void *stack)
{
	int ret;

#if defined(__hppa__)
	ret = clone(fn, stack, clone_flags, arg);
#elif defined(__ia64__)
	ret = clone2(fn, stack, stack_size, clone_flags, arg, NULL, NULL, NULL);
#elif defined(__arm__)
	/*
	 * Stack size should be a multiple of 32 bit words
	 * & stack limit must be aligned to a 32 bit boundary
	 */
	ret = clone(fn, (stack ? stack + stack_size : NULL),
			clone_flags, arg);
#else
	ret = clone(fn, (stack ? stack + stack_size - 1 : NULL),
			clone_flags, arg);
#endif

	return ret;
}


I believe the __arm__ case is to ensure that we pass an aligned
stack pointer to the syscall (as required by the architecture).
What I don't understand, is why we pass an unaligned address in
the #else case? Given that a stack push decrements the sp before
writing on every architecture I know of, it seems that the __arm__
case should be used as the default rather than forcing a misaligned
stack on other architectures.

Will




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Re: [LTP] ltp_clone

[Hannu Heikkinen]

On 25/02/11 11:11 +0000, ext Will Deacon wrote:
> Hello,
> 
> The definition of ltp_clone looks like this:
> 
> 
> int
> ltp_clone(unsigned long clone_flags, int (*fn)(void *arg), void *arg,
> 		size_t stack_size, void *stack)
> {
> 	int ret;
> 
> #if defined(__hppa__)
> 	ret = clone(fn, stack, clone_flags, arg);
> #elif defined(__ia64__)
> 	ret = clone2(fn, stack, stack_size, clone_flags, arg, NULL, NULL, NULL);
> #elif defined(__arm__)
> 	/*
> 	 * Stack size should be a multiple of 32 bit words
> 	 * & stack limit must be aligned to a 32 bit boundary
> 	 */
> 	ret = clone(fn, (stack ? stack + stack_size : NULL),
> 			clone_flags, arg);
> #else
> 	ret = clone(fn, (stack ? stack + stack_size - 1 : NULL),
> 			clone_flags, arg);
> #endif
> 
> 	return ret;
> }
> 
> 
> I believe the __arm__ case is to ensure that we pass an aligned
> stack pointer to the syscall (as required by the architecture).
> What I don't understand, is why we pass an unaligned address in
> the #else case? Given that a stack push decrements the sp before
> writing on every architecture I know of, it seems that the __arm__
> case should be used as the default rather than forcing a misaligned
> stack on other architectures.
> 
> Will
> 

Hi,

have you fetched the latest from the git tree:
git://ltp.git.sourceforge.net/gitroot/ltp/ltp ?

Lines are:

int
ltp_clone(unsigned long clone_flags, int (*fn)(void *arg), void *arg,
                size_t stack_size, void *stack)
{
        int ret;

#if defined(__hppa__)
        ret = clone(fn, stack, clone_flags, arg);
#elif defined(__ia64__)
        ret = clone2(fn, stack, stack_size, clone_flags, arg, NULL, NULL,
NULL);
#else
        /*
         * For archs where stack grows downwards, stack points to the topmost
         * address of the memory space set up for the child stack.
         */
        ret = clone(fn, (stack ? stack + stack_size : NULL),
                        clone_flags, arg);
#endif

        return ret;
}


br,
Hannu

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Re: [LTP] ltp_clone

[Will Deacon]

Hi Hannu,

> > I believe the __arm__ case is to ensure that we pass an aligned
> > stack pointer to the syscall (as required by the architecture).
> > What I don't understand, is why we pass an unaligned address in
> > the #else case? Given that a stack push decrements the sp before
> > writing on every architecture I know of, it seems that the __arm__
> > case should be used as the default rather than forcing a misaligned
> > stack on other architectures.
> >
> > Will
> >
> 
> Hi,
> 
> have you fetched the latest from the git tree:
> git://ltp.git.sourceforge.net/gitroot/ltp/ltp ?
> 
> Lines are:
> 
> int
> ltp_clone(unsigned long clone_flags, int (*fn)(void *arg), void *arg,
>                 size_t stack_size, void *stack)
> {
>         int ret;
> 
> #if defined(__hppa__)
>         ret = clone(fn, stack, clone_flags, arg);
> #elif defined(__ia64__)
>         ret = clone2(fn, stack, stack_size, clone_flags, arg, NULL, NULL,
> NULL);
> #else
>         /*
>          * For archs where stack grows downwards, stack points to the topmost
>          * address of the memory space set up for the child stack.
>          */
>         ret = clone(fn, (stack ? stack + stack_size : NULL),
>                         clone_flags, arg);
> #endif
> 
>         return ret;
> }


Wahey - that's much better! I was accidently looking in my checkout of the
October stable release, where ARM is special-cased.

Thanks for pointing this out.

Will




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