Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags

[Jan Kara <jack@suse.cz>]

On Sat 16-09-17 20:44:14, Dan Williams wrote:
> On Tue, Aug 15, 2017 at 5:27 AM, Jan Kara <jack@suse.cz> wrote:
> > On Mon 14-08-17 23:12:16, Dan Williams wrote:
> >> The mmap syscall suffers from the ABI anti-pattern of not validating
> >> unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT need a
> >> mechanism to define new behavior that is known to fail on older kernels
> >> without the feature. Use the fact that specifying MAP_SHARED and
> >> MAP_PRIVATE at the same time is invalid as a cute hack to allow a new
> >> set of validated flags to be introduced.
> >>
> >> This also introduces the ->fmmap() file operation that is ->mmap() plus
> >> flags. Each ->fmmap() implementation must fail requests when a locally
> >> unsupported flag is specified.
> > ...
> >> diff --git a/include/linux/fs.h b/include/linux/fs.h
> >> index 1104e5df39ef..bbe755d0caee 100644
> >> --- a/include/linux/fs.h
> >> +++ b/include/linux/fs.h
> >> @@ -1674,6 +1674,7 @@ struct file_operations {
> >>       long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
> >>       long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
> >>       int (*mmap) (struct file *, struct vm_area_struct *);
> >> +     int (*fmmap) (struct file *, struct vm_area_struct *, unsigned long);
> >>       int (*open) (struct inode *, struct file *);
> >>       int (*flush) (struct file *, fl_owner_t id);
> >>       int (*release) (struct inode *, struct file *);
> >> @@ -1748,6 +1749,12 @@ static inline int call_mmap(struct file *file, struct vm_area_struct *vma)
> >>       return file->f_op->mmap(file, vma);
> >>  }
> >>
> >> +static inline int call_fmmap(struct file *file, struct vm_area_struct *vma,
> >> +             unsigned long flags)
> >> +{
> >> +     return file->f_op->fmmap(file, vma, flags);
> >> +}
> >> +
> >
> > Hum, I dislike a new file op for this when the only problem with ->mmap is
> > that it misses 'flags' argument. I understand there are lots of ->mmap
> > implementations out there and modifying prototype of them all is painful
> > but is it so bad? Coccinelle patch for this should be rather easy...
> 
> So it wasn't all that easy, and Linus declined to take it. I think we
> should add a new ->mmap_validate() file operation and save the
> tree-wide cleanup until later.

Well, we don't even strictly need the flags passed to ->mmap callback if we
are willing to use VMA flags. I want to use it for MAP_SYNC anyway... So
bumping vma->flags to u64 and using a flag is also an option (and frankly
I'd personally just go for that).

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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