Re: [RFC PATCH 1/2] mm: introduce MAP_FIXED_SAFE

[Florian Weimer]

On 11/20/2017 10:33 AM, Michal Hocko wrote:
> 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.

On the glibc side, we see a lot of changes in error codes depending on 
kernel version, build and run-time configuration.  It never occurred to 
me that you guys think the precise error code is part of the userspace 
ABI.  Personally, I even assume that failure itself can disappear at any 
time (evidence: the f* functions which accept O_PATH in their non-*at 
variants).

Thanks,
Florian