Re: [Xenomai-core] [Xenomai-commits] r3147 - __xn_access_ok changes

[Philippe Gerum <rpm@xenomai.org>]

Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>  > [Let's discuss this without bothering users :)]
>  > 
>  > Philippe Gerum wrote:
>  > > Author: rpm
>  > > Date: Sun Nov  4 18:18:39 2007
>  > > New Revision: 3147
>  > > 
>  > > URL: http://svn.gna.org/viewcvs/xenomai?rev=3147&view=rev
>  > > Log:
>  > > Make __xn_access_ok() return false for addresses lower than the natural page size.
>  > > 
>  > > Modified:
>  > >     trunk/ChangeLog
>  > >     trunk/include/asm-x86/syscall_32.h
>  > >     trunk/include/asm-x86/syscall_64.h
>  > 
>  > Could it be that you meant "PAGE_SIZE" instead of "PAGE_OFFSET"? Because
>  > the current version is "slightly" broken, tagging any address in user
>  > land as invalid.
>  > 

Another example that committing last minute "fixes" before leaving for a 
trip is just as safe as checking for your parachute _after_ you jumped 
out of the plane. Fixed the trivial way for now. Sorry for this.

>  > And if this test was meant to catch NULL page accesses early, is the
>  > intention to cope with all those current i-pipe patches that do not yet
>  > include the discussed domain switch on non-root faults? If yes, this
>  > test would be a workaround for legacy code and should not become default
>  > (pure overhead for later versions).
> 
> We can reduce the overhead of the two tests by testing 
> (unsigned long) (addr - PAGE_SIZE) < (PAGE_OFFSET - PAGE_SIZE)
> 

Yep. I'd rather keep the reference to the actual task's segment limit in 
this expression though, instead of hardcoding PAGE_OFFSET.

Aside of this, we should not need to pass the current task pointer to 
each and every syscall anymore, so we may rely on the original uaccess code.

__xn_access_ok was there to allow for passing the task pointer
explicitly in order to test the address range against the task's segment
limit, at times when Adeos would switch the underlying stack to some
private, non-Linux one.

With modern I-pipe, domains don't have any private stack, but simply
reuse the current one, which means that any task running a syscall
over the Xenomai domain may always be referenced by "current", including
on archs using the stack pointer trick to determine this value.

I'll do this change early in the 2.5 series; it's too late for 2.4.

This said, we'll still need __xn_access_ok from now on, to pre-test the 
address for obvious spuriousness, like the NULL case which started this 
discussion.

>  > 
>  > Last question: We need this for the other archs as well, don't we?
>  > 

True. We also badly need to add the fixup code to the I-pipe for those 
archs.

>  > Jan
>  > 
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