Re: nommu: handling anonymous mmap clearing in userspace rather thankernel

[Bernd Schmidt]

Mike Frysinger wrote:
> On Wed, Apr 2, 2008 at 10:20 AM, David Howells <dhowells@xxxxxxxxxx> wrote:
> > Mike Frysinger <vapier.adi@xxxxxxxxx> wrote:
> >  > a workaround: introduce a new no-mmu-only mmap flag MAP_UNINITIALIZE
> >  > to signal to the kernel that it should skip the memset().  this way,
> >  > userspace malloc() can do mmap(MAP_ANONYMOUS|MAP_UNINITIALIZE) to get
> >  > large chunks of memory without affecting any other anonymous mmap()
> >  > call.
> >
> >  I think that's reasonable for NOMMU.  It's not like the process accessing the
> >  uninitialised memory is prevented from accessing anything it wants to anyway.
> >
> >  I would vote that the memset() should only be skipped if requested as there
> >  may be programs that call mmap(MAP_ANONYMOUS) expecting the memory they're
> >  given to be zeroed out.
>
> in the second proposal, the C library would be expected to do this, so
> no programs would be broken.  but you're right that any program that
> invokes the mmap() syscall directly would not get zeroed memory ...
> but is anyone doing such a crazy thing, let alone on embedded ?

It's not a guarantee we should break.  What's wrong with just using the 
MAP_UNINITIALIZE code we have?


Bernd
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