Re: mremap() use is racy

[Linus Torvalds]

On Tue, 23 Aug 2005, Ulrich Drepper wrote:
>
> Linus Torvalds wrote:
> > 
> > Especially if you use MAP_SHARED, you don't even need to mprotect 
> > anything: you'll get a nice SIGBUS if you ever try to access past the last 
> > page that maps the file.
> 
> If you guarantee this (and test for this) it's fine with me. 

It's how the kernel _should_ work, but very few apps seem to depend on it, 
so no guarantees. I looked over the code, and I think we've lost the 
SIGBUS thing.

Basically, if you don't ever access past the end, you should always be ok.  
If you access past the end, at least some _really_ old kernel versions
will zero-fill the page with an anonymous page and lose coherency, while
more modern kernels should _either_ cause a SIGBUS or alternatively at 
least have a coherent zero-filled page.

		Linus
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