Re: [PATCH v9 01/42] mm: Rename arch pte_mkwrite()'s to pte_mkwrite_novma()

["Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx>]

On Tue, 2023-06-13 at 10:43 +0300, Mike Rapoport wrote:
> On Mon, Jun 12, 2023 at 05:10:27PM -0700, Rick Edgecombe wrote:
> > The x86 Shadow stack feature includes a new type of memory called
> > shadow
> > stack. This shadow stack memory has some unusual properties, which
> > requires
> > some core mm changes to function properly.
> > 
> > One of these unusual properties is that shadow stack memory is
> > writable,
> > but only in limited ways. These limits are applied via a specific
> > PTE
> > bit combination. Nevertheless, the memory is writable, and core mm
> > code
> > will need to apply the writable permissions in the typical paths
> > that
> > call pte_mkwrite(). Future patches will make pte_mkwrite() take a
> > VMA, so
> > that the x86 implementation of it can know whether to create
> > regular
> > writable memory or shadow stack memory.
> 
> Nit:                            ^ mapping?

Hmm, sure.

> 
> > But there are a couple of challenges to this. Modifying the
> > signatures of
> > each arch pte_mkwrite() implementation would be error prone because
> > some
> > are generated with macros and would need to be re-implemented.
> > Also, some
> > pte_mkwrite() callers operate on kernel memory without a VMA.
> > 
> > So this can be done in a three step process. First pte_mkwrite()
> > can be
> > renamed to pte_mkwrite_novma() in each arch, with a generic
> > pte_mkwrite()
> > added that just calls pte_mkwrite_novma(). Next callers without a
> > VMA can
> > be moved to pte_mkwrite_novma(). And lastly, pte_mkwrite() and all
> > callers
> > can be changed to take/pass a VMA.
> > 
> > Start the process by renaming pte_mkwrite() to pte_mkwrite_novma()
> > and
> > adding the pte_mkwrite() wrapper in linux/pgtable.h. Apply the same
> > pattern for pmd_mkwrite(). Since not all archs have a
> > pmd_mkwrite_novma(),
> > create a new arch config HAS_HUGE_PAGE that can be used to tell if
> > pmd_mkwrite() should be defined. Otherwise in the !HAS_HUGE_PAGE
> > cases the
> > compiler would not be able to find pmd_mkwrite_novma().
> > 
> > No functional change.
> > 
> > Cc: linux-doc@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: linux-alpha@xxxxxxxxxxxxxxx
> > Cc: linux-snps-arc@xxxxxxxxxxxxxxxxxxx
> > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > Cc: linux-csky@xxxxxxxxxxxxxxx
> > Cc: linux-hexagon@xxxxxxxxxxxxxxx
> > Cc: linux-ia64@xxxxxxxxxxxxxxx
> > Cc: loongarch@xxxxxxxxxxxxxxx
> > Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
> > Cc: Michal Simek <monstr@xxxxxxxxx>
> > Cc: Dinh Nguyen <dinguyen@xxxxxxxxxx>
> > Cc: linux-mips@xxxxxxxxxxxxxxx
> > Cc: openrisc@xxxxxxxxxxxxxxxxxxxx
> > Cc: linux-parisc@xxxxxxxxxxxxxxx
> > Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> > Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
> > Cc: linux-s390@xxxxxxxxxxxxxxx
> > Cc: linux-sh@xxxxxxxxxxxxxxx
> > Cc: sparclinux@xxxxxxxxxxxxxxx
> > Cc: linux-um@xxxxxxxxxxxxxxxxxxx
> > Cc: linux-arch@xxxxxxxxxxxxxxx
> > Cc: linux-mm@xxxxxxxxx
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > Link:
> > https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@xxxxxxxxxxxxxx/
> 
> Reviewed-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>

Thanks!