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SubjectRe: nommu: handling anonymous mmap clearing in userspace rather than kernel
Mike Frysinger wrote:
> On Wed, Apr 2, 2008 at 10:20 AM, David Howells <dhowells@redhat.com> wrote:
>> Mike Frysinger <vapier.adi@gmail.com> 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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