Re: [PATCH v2 1/6] mm: introduce vma->vm_flags modifier functions

Matthew Wilcox
2023-01-25T18:37:36Z

On Wed, Jan 25, 2023 at 08:49:50AM -0800, Suren Baghdasaryan wrote:
> On Wed, Jan 25, 2023 at 1:10 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > + /*
> > > + * Flags, see mm.h.
> > > + * WARNING! Do not modify directly.
> > > + * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
> > > + */
> > > + unsigned long vm_flags;
> >
> > We have __private and ACCESS_PRIVATE() to help with enforcing this.
>
> Thanks for pointing this out, Peter! I guess for that I'll need to
> convert all read accesses and provide get_vm_flags() too? That will
> cause some additional churt (a quick search shows 801 hits over 248
> files) but maybe it's worth it? I think Michal suggested that too in
> another patch. Should I do that while we are at it?

Here's a trick I saw somewhere in the VFS:

union {
const vm_flags_t vm_flags;
vm_flags_t __private __vm_flags;
};

Now it can be read by anybody but written only by those using
ACCESS_PRIVATE.