Re: [PATCH v2 00/15] Avoid MAP_FIXED gap exposure

[Andrew Morton]

On Wed, 26 Jun 2024 21:15:18 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:

> * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> [240626 16:59]:
> > On Tue, 25 Jun 2024 15:11:30 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:
> > 
> > > It is now possible to walk the vma tree using the rcu read locks and is
> > > beneficial to do so to reduce lock contention.  Doing so while a
> > > MAP_FIXED mapping is executing means that a reader may see a gap in the
> > > vma tree that should never logically exist - and does not when using the
> > > mmap lock in read mode.  The temporal gap exists because mmap_region()
> > > calls munmap() prior to installing the new mapping.
> > 
> > What are the consequences when this race hits?  IOW, why do we need to
> > change anything?
> > 
> 
> In the (near) future, we want to walk the vma tree to produce
> /proc/<pid>/maps.  Without this change we will see the temporal gap and
> expose it to the user.  This series was initially sent to Suren as part
> of his patch set.
> 
> We also have the new interface for an ioctl request to a vma at or above
> an address. I had highlighted that an rcu reader would be ideal, but
> proved too difficult at this time. These patches by Andrii are currently
> not using the rcu reading method as this and a per-vma locking
> clarification are needed.
> 
> Since there were two users for this code, I decided to send it out
> before the other patches.

OK, thanks.  We're approaching -rc6 and things are a bit sketchy so I'm
inclined to hold this off until the next cycle, unless there's urgency?