Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

[Pavel Machek]

On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
> Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
> 
> > On Thu, Dec 07, 2017 at 11:14:27AM -0800, Kees Cook wrote:
> >> On Wed, Dec 6, 2017 at 9:46 PM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> >> > Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
> >> >> So, just like we currently say "exactly one of MAP_SHARED or MAP_PRIVATE",
> >> >> we could add a new paragraph saying "at most one of MAP_FIXED or
> >> >> MAP_REQUIRED" and "any of the following values".
> >> >
> >> > MAP_REQUIRED doesn't immediately grab me, but I don't actively dislike
> >> > it either :)
> >> >
> >> > What about MAP_AT_ADDR ?
> >> >
> >> > It's short, and says what it does on the tin. The first argument to mmap
> >> > is actually called "addr" too.
> >> 
> >> "FIXED" is supposed to do this too.
> >> 
> >> Pavel suggested:
> >> 
> >> MAP_ADD_FIXED
> >> 
> >> (which is different from "use fixed", and describes why it would fail:
> >> can't add since it already exists.)
> >> 
> >> Perhaps "MAP_FIXED_NEW"?
> >> 
> >> There has been a request to drop "FIXED" from the name, so these:
> >> 
> >> MAP_FIXED_NOCLOBBER
> >> MAP_FIXED_NOREPLACE
> >> MAP_FIXED_ADD
> >> MAP_FIXED_NEW
> >> 
> >> Could be:
> >> 
> >> MAP_NOCLOBBER
> >> MAP_NOREPLACE
> >> MAP_ADD
> >> MAP_NEW
> >> 
> >> and we still have the unloved, but acceptable:
> >> 
> >> MAP_REQUIRED
> >> 
> >> My vote is still for "NOREPLACE" or "NOCLOBBER" since it's very
> >> specific, though "NEW" is pretty clear too.
> >
> > How about MAP_NOFORCE?
> 
> It doesn't tell me that addr is not a hint. That's a crucial detail.
> 
> Without MAP_FIXED mmap never "forces/replaces/clobbers", so why would I
> need MAP_NOFORCE if I don't have MAP_FIXED?
> 
> So it needs something in there to indicate that the addr is not a hint,
> that's the only thing that flag actually *does*.
> 
> 
> If we had a time machine, the right set of flags would be:
> 
>   - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
>   - MAP_REPLACE: replace an existing mapping (or force or clobber)

Actually, if we had a time machine... would we even provide
MAP_REPLACE functionality?

> But the two were conflated for some reason in the current MAP_FIXED.
> 
> Given we can't go back and fix it, the closest we can get is to add a
> variant of MAP_FIXED which subtracts the "REPLACE" semantic.
> 
> ie: MAP_FIXED_NOREPLACE

I like MAP_FIXED_NOREPLACE.

									Pavel
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