Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag

[Michal Hocko]

On Wed 11-04-18 18:40:09, Jann Horn wrote:
> On Wed, Apr 11, 2018 at 6:36 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > On Wed 11-04-18 17:37:46, Jann Horn wrote:
> >> On Wed, Apr 11, 2018 at 2:04 PM,  <mhocko@xxxxxxxxxx> wrote:
> >> > From: Michal Hocko <mhocko@xxxxxxxx>
> >> >
> >> > 4.17+ kernels offer a new MAP_FIXED_NOREPLACE flag which allows the caller to
> >> > atomicaly probe for a given address range.
> >> >
> >> > [wording heavily updated by John Hubbard <jhubbard@xxxxxxxxxx>]
> >> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> >> > ---
> >> > Hi,
> >> > Andrew's sent the MAP_FIXED_NOREPLACE to Linus for the upcoming merge
> >> > window. So here we go with the man page update.
> >> >
> >> >  man2/mmap.2 | 27 +++++++++++++++++++++++++++
> >> >  1 file changed, 27 insertions(+)
> >> >
> >> > diff --git a/man2/mmap.2 b/man2/mmap.2
> >> > index ea64eb8f0dcc..f702f3e4eba2 100644
> >> > --- a/man2/mmap.2
> >> > +++ b/man2/mmap.2
> >> > @@ -261,6 +261,27 @@ Examples include
> >> >  and the PAM libraries
> >> >  .UR http://www.linux-pam.org
> >> >  .UE .
> >> > +Newer kernels
> >> > +(Linux 4.17 and later) have a
> >> > +.B MAP_FIXED_NOREPLACE
> >> > +option that avoids the corruption problem; if available, MAP_FIXED_NOREPLACE
> >> > +should be preferred over MAP_FIXED.
> >>
> >> This still looks wrong to me. There are legitimate uses for MAP_FIXED,
> >> and for most users of MAP_FIXED that I'm aware of, MAP_FIXED_NOREPLACE
> >> wouldn't work while MAP_FIXED works perfectly well.
> >>
> >> MAP_FIXED is for when you have already reserved the targeted memory
> >> area using another VMA; MAP_FIXED_NOREPLACE is for when you haven't.
> >> Please don't make it sound as if MAP_FIXED is always wrong.
> >
> > Well, this was suggested by John. I think, nobody is objecting that
> > MAP_FIXED has legitimate usecases. The above text just follows up on
> > the previous section which emphasises the potential memory corruption
> > problems and it suggests that a new flag is safe with that regards.
> >
> > If you have specific wording that would be better I am open for changes.
> 
> I guess I'd probably also want to change the previous text; so I
> should probably send a followup patch once this one has landed.

yeah, that sounds like a better plan.
-- 
Michal Hocko
SUSE Labs