Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

[Lorenzo Stoakes]

On Fri, Jul 25, 2025 at 11:46:13AM +0200, David Hildenbrand wrote:
> > >
> > > Well, there is long-term pinning that can break COW and other weird stuff
> > > like FOLL_FORCE. Most of the latter probably holds the mmap lock in write
> > > mode. Probably.
> >
> > Well GUP uses read lock.
>
> Right, so it can race with MADV_DONTNEED.
>
> >
> > FOLL_FORCE won't override anything as we have this check in check_vma_flags():
> >
> > 	if (write) {
> > 		if (!vma_anon &&
> > 		    !writable_file_mapping_allowed(vma, gup_flags))
> > 			return -EFAULT;
> >
> > 		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
> > 			if (!(gup_flags & FOLL_FORCE))
> > 				return -EFAULT;
> > 			/*
> > 			 * We used to let the write,force case do COW in a
> > 			 * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
> > 			 * set a breakpoint in a read-only mapping of an
> > 			 * executable, without corrupting the file (yet only
> > 			 * when that file had been opened for writing!).
> > 			 * Anon pages in shared mappings are surprising: now
> > 			 * just reject it.
> > 			 */
> > 			if (!is_cow_mapping(vm_flags))
> > 				return -EFAULT;
> > 		}
> > 	}
> >
> > With:
> >
> > static inline bool is_cow_mapping(vm_flags_t flags)
> > {
> > 	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
> > }
> >
>
> Not sure what you mean. Using FOLL_FORCE you can write into MAP_PRIVATE R/O
> mappings. Particular useful for installing breakpoints into loaded
> executables etc.

Sigh. Really sorry, I'm having a terrible week, my brain just isn't working at
the moment.

Yes it proves the opposite of what I said, I misread it foolishly.

Please disregard.

>
> is_cow_mapping() tells you exactly that: the only place where we can have
> anon folios is when we have a MAP_PRIVATE mapping (!VM_SHARED) that can be
> writable, for example, through mprotect(PROT_WRITE) (VM_MAYWRITE).
>
> A MAP_PRIVATE R/O file mapping matches is_cow_mapping().

Yup.

>
> > So - we explicitly disallow FOLL_FORCE write override for CoW file-backed
> > mappings.
> >
> > Obviously if FOLL_FORCE is not set, then we're ALSO not allowed to get past a
> > FOLL_WRITE and !VM_WRITE situation.
> >
> > >
> > > >
> > > > Hmm maybe I'll soften on this anon_vma idea then. Maybe it is a 'cheap fix' to
> > > > rule out the _usual_ cases.
> > >
> > > Yeah, something to evaluate.
> >
> > I'm thinking more and more we're probably actually safe with !vma->anon_vma ||
> > !(vma->vm_flags & VM_MAYWRITE).
>
> I think there are possible races, the question is how much you care about
> them.

Yes I was just wrong. Please just disregard.

I mean racing with MADV_POPULATE_WRITE seems a niche thing to worry about, and
so what if you did, it's writing a... copy of the underlying file-backed folios
no?

Equally long-term GUP, assuming it breaks CoW for migration, is what, populating
unchanged folios, so what's the issue?


>
> In case of CoW-unsharing, you're not actually discarding data, because the
> page in the anon folio is to maintain a copy of the pagecache page (of
> course, they can go out of sync, but that's a different discussion).

Yes I know.