linux-kernel - Re: [PATCH 02/15] arm: __NR

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Message-ID: <20140423102718.GA6434@n2100.arm.linux.org.uk>
Date:	Wed, 23 Apr 2014 11:27:18 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>,
	Wade Farnsworth <wade_farnsworth@...tor.com>,
	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-arch <linux-arch@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>
Subject: Re: [PATCH 02/15] arm: __NR_syscalls fix

On Fri, Apr 11, 2014 at 01:45:02PM +0100, Russell King - ARM Linux wrote:
> On Fri, Apr 11, 2014 at 02:02:32PM +0200, Miklos Szeredi wrote:
> > On Fri, Apr 11, 2014 at 1:54 PM, Russell King - ARM Linux
> > <linux@....linux.org.uk> wrote:
> > > On Fri, Apr 11, 2014 at 01:50:50PM +0200, Miklos Szeredi wrote:
> > >> Yes, and the padding will be of wrong length if NR_syscalls is
> > >> incorrect (which may be Oopsable?).  At least that is my impression
> > >> from a casual glance.
> > >
> > > Please explain.
> > 
> > Look at ending lines of arch/arm/kernel/calls.S: if  NR_syscalls is a
> > multiple of 4, then syscalls_padding will be zero.  I.e. no padding
> > despite the fact that there is in fact only 382 system calls in table
> > and there should be 2 sys_ni_syscall pads.
> 
> Hmm, it looks like you're right... this used to work fine until...
> 
> commit 1f66e06fb6414732bef7bf4a071ef76a837badec
> Author: Wade Farnsworth <wade_farnsworth@...tor.com>
> Date:   Fri Sep 7 18:18:25 2012 +0100
> 
>     ARM: 7524/1: support syscall tracing
> 
> because the tracing code wanted to know the number of syscalls.  I don't
> know what the answer is here, because the current solution is IMHO far
> to fragile.

Actually, no, you're wrong.  Look closely at the definitions. __NR_syscalls
is not the same as NR_syscalls.

__NR_syscalls is the statically defined size of the syscall table for
*probes purposes.

NR_syscalls is the assembly-counted number of CALL() macros in
arch/arm/kernel/calls.S.

So, patch 2 isn't required, and patch 3 needs to be fixed up to take
this into account...

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