Re: [PATCH] fanotify: Fix sys_fanotify_mark() on native x86-32

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Dec 01, 2020 at 09:34:32AM -0800, Andy Lutomirski wrote:
> On Tue, Dec 1, 2020 at 9:23 AM Andy Lutomirski <luto@kernel.org> wrote:
> > On Mon, Nov 30, 2020 at 2:31 PM Brian Gerst <brgerst@gmail.com> wrote:
> > > Commit 121b32a58a3a converted native x86-32 which take 64-bit arguments to
> > > use the compat handlers to allow conversion to passing args via pt_regs.
> > > sys_fanotify_mark() was however missed, as it has a general compat handler.
> > > Add a config option that will use the syscall wrapper that takes the split
> > > args for native 32-bit.
> > >
> > > Reported-by: Paweł Jasiak <pawel@jasiak.xyz>
> > > Fixes: 121b32a58a3a ("x86/entry/32: Use IA32-specific wrappers for syscalls taking 64-bit arguments")
> > > Signed-off-by: Brian Gerst <brgerst@gmail.com>
> > > ---
> > >  arch/Kconfig                       |  6 ++++++
> > >  arch/x86/Kconfig                   |  1 +
> > >  fs/notify/fanotify/fanotify_user.c | 17 +++++++----------
> > >  include/linux/syscalls.h           | 24 ++++++++++++++++++++++++
> > >  4 files changed, 38 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/arch/Kconfig b/arch/Kconfig
> > > index 090ef3566c56..452cc127c285 100644
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -1045,6 +1045,12 @@ config HAVE_STATIC_CALL_INLINE
> > >         bool
> > >         depends on HAVE_STATIC_CALL
> > >
> > > +config ARCH_SPLIT_ARG64
> > > +       bool
> > > +       help
> > > +          If a 32-bit architecture requires 64-bit arguments to be split into
> > > +          pairs of 32-bit arguemtns, select this option.
> >
> > You misspelled arguments.  You might also want to clarify that, for
> > 64-bit arches, this means that compat syscalls split their arguments.
> 
> No, that's backwards.  Maybe it should be depends !64BIT instead.
> 
> But I'm really quite confused about something: what's special about
> x86 here?  Are there really Linux arches (compat or 32-bit native)
> that *don't* split arguments like this?  Sure, some arches probably
> work the same way that x86 used to in which the compiler did the
> splitting by magic for us, but that was always a bit of a kludge.

On arm32 we rely on the compiler splitting a 64-bit argument in two
consecutive registers. But I wouldn't say it's a kludge (well, mostly)
as that's part of the arm procedure calling standard. Currently arm32
doesn't pass the syscall arguments through a read from pt_regs, so all
is handled transparently.

On arm64 compat, we need to re-assemble the arguments with some
wrappers explicitly (arch/arm64/kernel/sys32.c) or call the generic
wrapper like in the compat_sys_fanotify_mark() case.

> Could this change maybe be made unconditional?

I think it's fine in this particular case.

I don't think it's valid in general because of the arm (and maybe
others) requirement that the first register of a 64-bit argument is an
even number (IIRC, Russell should know better). If the u64 mask was an
argument before or after the current position, the compiler would have
introduced a pad register but not if the arg is split in two u32.

-- 
Catalin