Re: [PATCH v7 4/7] mseal, system mappings: enable arm64

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [250226 00:26]:
> On Tue, Feb 25, 2025 at 02:26:50PM -0800, Jeff Xu wrote:
> > On Mon, Feb 24, 2025 at 10:20 PM Lorenzo Stoakes
> > <lorenzo.stoakes@oracle.com> wrote:
> > >
> > > On Mon, Feb 24, 2025 at 10:52:43PM +0000, jeffxu@chromium.org wrote:
> > > > From: Jeff Xu <jeffxu@chromium.org>
> > > >
> > > > Provide support for CONFIG_MSEAL_SYSTEM_MAPPINGS on arm64, covering
> > > > the vdso, vvar, and compat-mode vectors and sigpage mappings.
> > > >
> > > > Production release testing passes on Android and Chrome OS.
> > >
> > > This is pretty limited (yes yes I know android is massive etc. but we must
> > > account for all the weird and wonderful arm64 devices out there in context of
> > > upstream :)
> > >
> > > Have you looking through all arm64-code relating to vdso, vvar, compat-mode
> > > vectors, sigpage mapping and ensured nothing kernel-side relies upon relocation?
> > > Some arches actually seem to want to do this. Pretty sure PPC does... so a bit
> > > nervous of that.
> > >
> > Can you please point out where PPC munmap/mremap the vdso ?
> >
> > Previously, when you mentioned that, I thought you meant user space in
> > PPC, I didn't realize that you meant that kernel code in PPC.  I
> > tried, but didn't find anything, hence asking.
> 
> Jeff, please stick to replying to review. 'Have you looking through all
> arm64-code'.
> 
> I ended up doing this myself yesterday and found no issues, as with x86-64.
> 
> I said I'm _pretty sure_ PPC does this. Liam mentioned something about
> it. We can discuss it, and I can find specifics if + when you try to add
> this to PPC.
> 

PPC allows the vma to be munmapped then detects and falls back to the
slower method, iirc.

They were against the removal of the fallback; other archs also have
this infrastructure.  Really, if we fixed the fallback to work for
all platforms then it would probably also remove the possibility of a
remap over the VDSO being a problem (if it is today, which still isn't
clear?).

Thanks,
Liam