Re: [PATCH v5 13/25] m68k: add asm/syscall.h

["Dmitry V. Levin" <ldv@altlinux.org>]

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Hi Geert,

On Mon, Dec 10, 2018 at 02:06:28PM +0100, Geert Uytterhoeven wrote:
> On Mon, Dec 10, 2018 at 1:41 PM Dmitry V. Levin <ldv@altlinux.org> wrote:
> > On Mon, Dec 10, 2018 at 09:45:42AM +0100, Geert Uytterhoeven wrote:
> > > On Mon, Dec 10, 2018 at 5:30 AM Dmitry V. Levin <ldv@altlinux.org> wrote:
> > > > syscall_get_* functions are required to be implemented on all
> > > > architectures in order to extend the generic ptrace API with
> > > > PTRACE_GET_SYSCALL_INFO request.
> > > >
> > > > This introduces asm/syscall.h on m68k implementing all 5 syscall_get_*
> > > > functions as documented in asm-generic/syscall.h: syscall_get_nr,
> > > > syscall_get_arguments, syscall_get_error, syscall_get_return_value,
> > > > and syscall_get_arch.
> > > >
> > > > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > > Cc: Andy Lutomirski <luto@kernel.org>
> > > > Cc: Elvira Khabirova <lineprinter@altlinux.org>
> > > > Cc: Eugene Syromyatnikov <esyr@redhat.com>
> > > > Cc: linux-m68k@lists.linux-m68k.org
> > > > Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
> > > > ---
> > > >
> > > > Notes:
> > > >     v5: added syscall_get_nr, syscall_get_arguments, syscall_get_error,
> > > >         and syscall_get_return_value
> > > >     v1: added syscall_get_arch
> > >
> > > > --- /dev/null
> > > > +++ b/arch/m68k/include/asm/syscall.h
> > > > @@ -0,0 +1,39 @@
> > >
> > > > +static inline void
> > > > +syscall_get_arguments(struct task_struct *task, struct pt_regs *regs,
> > > > +                     unsigned int i, unsigned int n, unsigned long *args)
> > > > +{
> > > > +       BUG_ON(i + n > 6);
> > >
> > > Does this have to crash the kernel?
> >
> > This is what most of other architectures do, but we could choose
> > a softer approach, e.g. use WARN_ON_ONCE instead.
> >
> > > Perhaps you can return an error code instead?
> >
> > That would be problematic given the signature of this function
> > and the nature of the potential bug which would most likely be a usage error.
> 
> Of course to handle that, the function's signature need to be changed.
> Changing it has the advantage that the error handling can be done at the
> caller, in common code, instead of duplicating it for all
> architectures, possibly
> leading to different semantics.

Given that *all* current users of syscall_get_arguments specify i == 0
(and there is an architecture that has BUG_ON(i)), 
it should be really a usage error to get into situation where i + n > 6,
I wish a BUILD_BUG_ON could be used here instead.

I don't think it worths pushing the change of API just to convert
a "cannot happen" assertion into an error that would have to be dealt with
on the caller side.


-- 
ldv

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