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

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Date:	Mon, 24 Aug 2009 20:34:32 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mundt <lethal@...ux-sh.org>, Jason Baron <jbaron@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, 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


* Paul Mundt <lethal@...ux-sh.org> wrote:

> On Mon, Aug 24, 2009 at 10:42:28AM -0400, Jason Baron wrote:
> > 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...
> 
>
> If FTRACE_SYSCALL_MAX is dropped then s390 will be fixed, and I'll 
> take care of the sh update. If you want to hold off on adding 
> NR_syscalls back to x86, then s390 will need a #define 
> FTRACE_SYSCALL_MAX __NR_syscalls in 
> arch/s390/include/asm/ftrace.h. Keeping FTRACE_SYSCALL_MAX around 
> seems to be asking for trouble, though (although I don't know what 
> the original rationale behind adding it was).

I agree with you - we should certainly add a clean and arch-generic 
way and drop the FTRACE_SYSCALL_MAX hack which really just tried to 
hide the arch differences for no strong reason.

At the same time the compat syscall space should be solved too, and 
a synonymous compat_NR_syscalls value introduced. (perhaps defined 
to 0 on non-compat kernels)

	Ingo
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