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

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On Fri, Nov 27, 2020 at 2:30 PM Brian Gerst <brgerst@xxxxxxxxx> wrote:
>
> On Fri, Nov 27, 2020 at 1:13 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > On Thu, Nov 26, 2020 at 7:52 AM Jan Kara <jack@xxxxxxx> 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 also wonder if there's a straightforward way to statically check
this.  Maybe the syscall wrapper macros could be rigged up to avoid
emitting the ia32 stubs if there is a u64 or s64 arg, so the build
would fail if someone tries to stick one in the syscall tables.  I
tried to do this, but I got a bit lost in the macro maze and my
attempt didn't work.

--Andy



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