linux-kernel - Re: [PATCH RT] Fix NR

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Date:	Sat, 2 Jun 2007 00:27:56 +0100
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Deepak Saxena <dsaxena@...xity.net>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RT] Fix NR_syscalls in ARM

On Sat, Jun 02, 2007 at 12:18:40AM +0100, Russell King wrote:
> On Fri, Jun 01, 2007 at 04:10:53PM -0700, Deepak Saxena wrote:
> > The -rt patch adds a NR_syscalls symbol to the arm/unistd.h but
> > it is not the correct value as there are 348 syscalls on ARM
> > and the existing change sets the symbol to 322.
> > 
> > Russell: Why isn't this in mainline? Other arches all seem to have 
> > this symbol already defined.
> 
> The hint is that it isn't in mainline; it's just plainly not required.
> It's also the wrong place to define it; it's not a property that
> unistd.h should concern itself with - it's a property of the kernel's
> branch table for calling the syscalls, and on ARM we calculate that
> number directly from the size of the kernel's branch table.
> 
> It's also not just last_syscall_number+1 since the table is sized to
> make the assembly easy - iow, a number divisible by 4.
> 
> So all in all, NR_syscalls in unistd.h is just utterly wrong.

BTW, it should be pointed out that you've found the exact reason why
putting it in unistd.h is _wrong_.  It's all to easy for it to get
out of sync with updates to the place where it really matters - the
code which bounds-checks the syscall number (that being the assembly
code which indexes the branch table.)

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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