Re: [PATCH] fanotify: Fix fanotify_mark() on 32-bit x86

[Brian Gerst <brgerst@gmail.com>]

On Fri, Nov 27, 2020 at 7:36 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Fri, Nov 27, 2020 at 2:30 PM Brian Gerst <brgerst@gmail.com> wrote:
> >
> > On Fri, Nov 27, 2020 at 1:13 PM Andy Lutomirski <luto@kernel.org> wrote:
> > >
> > > On Thu, Nov 26, 2020 at 7:52 AM Jan Kara <jack@suse.cz> wrote:
> > > >
> > > > Commit converting syscalls taking 64-bit arguments to new scheme of compat
> > > > handlers omitted converting fanotify_mark(2) which then broke the
> > > > syscall for 32-bit x86 builds. Add missed conversion. It is somewhat
> > > > cumbersome since we need to keep the original compat handler for all the
> > > > other 32-bit archs.
> > > >
> > >
> > > This is stupendously ugly.  I'm not really sure how this is supposed
> > > to work on any 32-bit arch.  I'm also not sure whether we should
> > > expect the SYSCALL_DEFINE macros to figure this out by themselves.
> >
> > It works on 32-bit arches because the compiler implicitly uses
> > consecutive input registers or stack slots for 64-bit arguments, and
> > some arches have alignment requirements that result in hidden padding.
> > x86-32 is different now because parameters are passed in via pt_regs,
> > and the 64-bit value has to explicitly be reassembled from the high
> > and low 32-bit values, just like in the compat case.
> >
>
> That was my guess.
>
> > I think the simplest way to handle this is add a wrapper in
> > arch/x86/kernel/sys_ia32.c with the other fs syscalls that need 64-bit
> > args.  That keeps this mess out of general code.
>
>
> Want to send a patch?

I settled on doing something along the same line as Jan, but in a more
generic way that lays the groundwork for converting more of these
arch-specific compat wrappers to a generic wrapper.

Patch coming soon.

--
Brian Gerst