Re: [RFC PATCH v3 11/11] mseal:add documentation

[Pedro Falcato]

On Thu, Dec 14, 2023 at 6:07 PM Stephen Röttger <sroettger@xxxxxxxxxx> wrote:
>
> On Thu, Dec 14, 2023 at 2:31 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Wed, 13 Dec 2023 at 16:36, Jeff Xu <jeffxu@xxxxxxxxxx> wrote:
> > >
> > >
> > > > IOW, when would you *ever* say "seal this area, but MADV_DONTNEED is ok"?
> > > >
> > > The MADV_DONTNEED is OK for file-backed mapping.
> >
> > Right. It makes no semantic difference. So there's no point to it.
> >
> > My point was that you added this magic flag for "not ok for RO anon mapping".
> >
> > It's such a *completely* random flag, that I go "that's just crazy
> > random - make sealing _always_ disallow that case".
> >
> > So what I object to in this series is basically random small details
> > that should just eb part of the basic act of sealing.
> >
> > I think sealing should just mean "you can't do any operations that
> > have semantic meaning for the mapping, because it is SEALED".
> >
> > So I think sealing should automatically mean "can't do MADV_DONTNEED
> > on anon memory", because that's basically equivalent to a munmap/remap
> > operation.
>
> In Chrome, we have a use case to allow MADV_DONTNEED on sealed memory.

I don't want to be that guy (*believe me*), but what if there was a
way to attach BPF programs to mm's? Such that you could handle 'seal
failures' in BPF, and thus allow for this sort of weird semantics?
e.g: madvise(MADV_DONTNEED) on a sealed region fails, kernel invokes
the BPF program (that chrome loaded), BPF program sees it was a
MADV_DONTNEED and allows it to proceed.

It requires BPF but sounds like a good compromise in order to not get
an ugly API?

-- 
Pedro