Re: do_mmap

[torvalds@transmeta.com (Linus Torvalds)]

In article <1022855243.12888.410.camel@irongate.swansea.linux.org.uk>,
Alan Cox  <alan@lxorguk.ukuu.org.uk> wrote:
>On Fri, 2002-05-31 at 14:00, Thomas 'Dent' Mirlacher wrote:
>> and the checks in various places are really strange. - well some
>> places check for:
>> 	o != NULL
>> 	o > -1024UL
>
>"Not an error". Its relying as some other bits of code do actually that
>the top mappable user address is never in the top 1K of the address
>space
>
>> is it possible to have 0 as a valid address? - if not, this should
>> be the return on errors.
>
>SuS explicitly says that 0 is not a valid mmap return address.

But if so, SuS is _very_ _very_ wrong.

The fact is, if you use something like vm86 mode, you absolutely _need_
to be able to explicitly mmap at address 0. 

So it is correct (and in fact there is no other sane way to do it) to
say

	addr = mmap(NULL, 1024*1024,
		PROT_READ | PROT_WRITE ,
		MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED,
		-1, 0);

and if SuS says that mmap must not return NULL for this case, then SuS
is so full of sh*t that it's not worth worrying about.

In short, under Linux 0 _is_, and will always be (at least on x86) a
perfectly valid return address from mmap() and friends. It's only going
to be returned when you explicitly ask for it with MAP_FIXED, but it
absolutely is a valid return.

		Linus
		Linus