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

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Message-ID: <20190329220425.GA3563@altlinux.org>
Date:   Sat, 30 Mar 2019 01:04:26 +0300
From:   "Dmitry V. Levin" <ldv@...linux.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Elvira Khabirova <lineprinter@...linux.org>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 13/25] m68k: add asm/syscall.h

On Wed, Dec 12, 2018 at 11:55:16AM +0300, Dmitry V. Levin wrote:
> On Mon, Dec 10, 2018 at 04:30:25PM +0300, Dmitry V. Levin wrote:
> > 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@...linux.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@...linux.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@...ux-m68k.org>
> > > > > > Cc: Oleg Nesterov <oleg@...hat.com>
> > > > > > Cc: Andy Lutomirski <luto@...nel.org>
> > > > > > Cc: Elvira Khabirova <lineprinter@...linux.org>
> > > > > > Cc: Eugene Syromyatnikov <esyr@...hat.com>
> > > > > > Cc: linux-m68k@...ts.linux-m68k.org
> > > > > > Signed-off-by: Dmitry V. Levin <ldv@...linux.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.
> 
> I suggest the following BUG_ON replacement for syscall_get_arguments:
> 
> #define SYSCALL_MAX_ARGS 6
> 
> static inline void
> syscall_get_arguments(struct task_struct *task, struct pt_regs *regs,
> 		      unsigned int i, unsigned int n, unsigned long *args)
> {
> 	/*
> 	 * Ideally there should have been
> 	 * BUILD_BUG_ON(i + n > SYSCALL_MAX_ARGS);
> 	 * instead of these checks.
> 	 */
> 	if (unlikely(i > SYSCALL_MAX_ARGS)) {
> 		WARN_ONCE(1, "i > SYSCALL_MAX_ARGS");
> 		return;
> 	}
> 	if (unlikely(n > SYSCALL_MAX_ARGS - i)) {
> 		WARN_ONCE(1, "i + n > SYSCALL_MAX_ARGS");
> 		n = SYSCALL_MAX_ARGS - i;
> 	}
> 	BUILD_BUG_ON(sizeof(regs->d1) != sizeof(args[0]));
> 	memcpy(args, &regs->d1 + i, n * sizeof(args[0]));
> }

There seems to be a more straightforward approach to this issue.

Assuming there is a general consensus [1] to get rid of "i" and "n"
arguments of syscall_get_arguments(), the implementation could be
simplified to

static inline void
syscall_get_arguments(struct task_struct *task, struct pt_regs *regs,
		      unsigned long *args)
{
	memcpy(args, &regs->d1, 6 * sizeof(args[0]));
}

[1] https://lore.kernel.org/lkml/20190328230512.486297455@goodmis.org/


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