linux-kernel - Re: [PATCH 05/12] update FTRACE_SYSCALL

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Date:	Mon, 24 Aug 2009 10:42:28 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Paul Mundt <lethal@...ux-sh.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org,
	mathieu.desnoyers@...ymtl.ca, jiayingz@...gle.com,
	mbligh@...gle.com, lizf@...fujitsu.com
Subject: Re: [PATCH 05/12] update FTRACE_SYSCALL_MAX

On Mon, Aug 24, 2009 at 11:15:39PM +0900, Paul Mundt wrote:
> On Mon, Aug 24, 2009 at 10:06:29AM -0400, Jason Baron wrote:
> > On Mon, Aug 24, 2009 at 10:41:52PM +0900, Paul Mundt wrote:
> > > I hope you can clarify what the meaning of this is supposed to be
> > > exactly. Is this number supposed to be the last usable syscall, or is it
> > > supposed to be the equivalent of NR_syscalls?
> > > 
> > 
> > I am using as the equivalent of NR_syscalls.
> > 
> NR_syscalls has always been the total number of system calls, not the
> last one.
> 
> > > Presently on SH we have this as NR_syscalls - 1, while on s390 I see it
> > > is treated as NR_syscalls directly. s390 opencodes the NR_syscalls
> > > directly and so presently blows up in -next due to a missing
> > > FTRACE_SYSCALL_MAX definition:
> > > 
> > > 	http://kisskb.ellerman.id.au/kisskb/buildresult/1120523/
> > > 
> > > I was in the process of fixing that up when I noticed this difference.
> > > x86 seems to also treat this as NR_syscalls - 1, but that looks to me
> > > like there is an off-by-1 in arch_init_ftrace_syscalls() causing the last
> > > syscall to be skipped?
> > 
> > I don't see how its used as 'NR_syscalls - 1' on x86,
> > arch_init_ftrace_syscalls() does:
> > 
> >         for (i = 0; i < FTRACE_SYSCALL_MAX; i++) {
> >                 meta = find_syscall_meta(psys_syscall_table[i]);
> >                 syscalls_metadata[i] = meta;
> >         }
> > 
> > So the last syscall should not be skipped.
> > 
> 
> In today's -next:
> 
> #ifdef CONFIG_X86_64
> # define FTRACE_SYSCALL_MAX     299
> #else
> # define FTRACE_SYSCALL_MAX     337
> #endif
> 
> unistd_32.h:
> 
> #define __NR_reflinkat          337
> 
> unistd_64.h:
> 
> #define __NR_reflinkat          299
> 
> The first syscall starts at 0, but I don't see how this last syscall is
> handled. If there were a __NR_syscalls 300 and 338 respectively, that
> would seem to do the right thing. Or am I missing something?

No, you are right. When I changed the FTRACE_SYSCALL_MAX to 299, and
337, there was no reflinkat syscall in the tree. So, it was equivalent
to NR_syscalls at that point in time. So that's where the confusion is.

Clearly, all the more reason to drop FTRACE_SYSCALL_MAX and change to
NR_syscalls...

thanks,

-Jason

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