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

[Michal Hocko]

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.

> > +.TP
> > +.BR MAP_FIXED_NOREPLACE " (since Linux 4.17)"
> > +Similar to MAP_FIXED with respect to the
> > +.I
> > +addr
> > +enforcement, but different in that MAP_FIXED_NOREPLACE never clobbers a pre-existing
> > +mapped range. If the requested range would collide with an existing
> > +mapping, then this call fails with
> > +.B EEXIST.
> > +This flag can therefore be used as a way to atomically (with respect to other
> > +threads) attempt to map an address range: one thread will succeed; all others
> > +will report failure. Please note that older kernels which do not recognize this
> > +flag will typically (upon detecting a collision with a pre-existing mapping)
> > +fall back to a "non-MAP_FIXED" type of behavior: they will return an address that
> > +is different than the requested one. Therefore, backward-compatible software
> > +should check the returned address against the requested address.
> >  .TP
> >  .B MAP_GROWSDOWN
> >  This flag is used for stacks.
> > @@ -487,6 +508,12 @@ is not a valid file descriptor (and
> >  .B MAP_ANONYMOUS
> >  was not set).
> >  .TP
> > +.B EEXIST
> > +range covered by
> > +.IR addr ,
> > +.IR length
> > +is clashing with an existing mapping.
> 
> Maybe add something like ", and MAP_FIXED_NOREPLACE was specified"? I
> think most manpages explicitly document which error conditions can be
> triggered by which flags.

sure, no objection from me.

-- 
Michal Hocko
SUSE Labs