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SubjectRe: [RFC PATCH 1/2] mm: introduce MAP_FIXED_SAFE
On Mon 20-11-17 10:10:32, Florian Weimer wrote:
> On 11/20/2017 09:55 AM, Michal Hocko wrote:
> > On Fri 17-11-17 08:30:48, Florian Weimer wrote:
> > > On 11/16/2017 11:18 AM, Michal Hocko wrote:
> > > > + if (flags & MAP_FIXED_SAFE) {
> > > > + struct vm_area_struct *vma = find_vma(mm, addr);
> > > > +
> > > > + if (vma && vma->vm_start <= addr)
> > > > + return -ENOMEM;
> > > > + }
> > >
> > > Could you pick a different error code which cannot also be caused by a an
> > > unrelated, possibly temporary condition? Maybe EBUSY or EEXIST?
> >
> > Hmm, none of those are described in the man page. I am usually very
> > careful to not add new and potentially unexpected error codes but it is
>
> I think this is a bad idea. It leads to bizarre behavior, like open failing
> with EOVERFLOW with certain namespace configurations (which have nothing to
> do with file sizes).

Ohh, I agree but breaking userspace is, you know, no-no. And an
unexpected error codes can break things terribly.

> Most of the manual pages are incomplete regarding error codes, and with
> seccomp filters and security modules, what error codes you actually get is
> anyone's guess.
>
> > true that a new flag should warrant a new error code. I am not sure
> > which one is more appropriate though. EBUSY suggests that retrying might
> > help which is true only if some other party unmaps the range. So EEXIST
> > would sound more natural.
>
> Sure, EEXIST is completely fine.

OK, I will use it.

> > > This would definitely help with application-based randomization of mappings,
> > > and there, actual ENOMEM and this error would have to be handled
> > > differently.
> >
> > I see. Could you be more specific about the usecase you have in mind? I
> > would incorporate it into the patch description.
>
> glibc ld.so currently maps DSOs without hints. This means that the kernel
> will map right next to each other, and the offsets between them a completely
> predictable. We would like to change that and supply a random address in a
> window of the address space. If there is a conflict, we do not want the
> kernel to pick a non-random address. Instead, we would try again with a
> random address.

This makes sense to me. Thanks, I will add it to the cover letter.
--
Michal Hocko
SUSE Labs

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