linux-kernel - Re: [PATCH v3] kernel/signal.c: fix BUG

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Date:	Wed, 26 Jun 2013 19:15:51 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
Cc:	James Hogan <james.hogan@...tec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Daney <ddaney@...iumnetworks.com>,
	David Daney <ddaney.cavm@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	David Daney <david.daney@...ium.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>, linux-mips@...ux-mips.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v3] kernel/signal.c: fix BUG_ON with SIG128 (MIPS)

On 06/26, Ralf Baechle wrote:
>
> On Wed, Jun 26, 2013 at 06:14:52PM +0200, Oleg Nesterov wrote:
>
> > Or simply remove the BUG_ON(), this can equally confuse wait(status).
> > 128 & 0x7f == 0.
> >
> > Still I think it would be better to change _NSIG on mips.
>
> If it was that easy.  That's going to outright break binary compatibility,
> see kernel/signal.c:
>
> SYSCALL_DEFINE4(rt_sigprocmask, int, how, sigset_t __user *, nset,
>                 sigset_t __user *, oset, size_t, sigsetsize)
> {
>         sigset_t old_set, new_set;
>         int error;
>
>         /* XXX: Don't preclude handling different sized sigset_t's.  */
>         if (sigsetsize != sizeof(sigset_t))
>                 return -EINVAL;

I meant the minimal hack like

	--- x/arch/mips/include/uapi/asm/signal.h
	+++ x/arch/mips/include/uapi/asm/signal.h
	@@ -11,9 +11,9 @@
	 
	 #include <linux/types.h>
	 
	-#define _NSIG		128
	+#define _NSIG		127
	 #define _NSIG_BPW	(sizeof(unsigned long) * 8)
	-#define _NSIG_WORDS	(_NSIG / _NSIG_BPW)
	+#define _NSIG_WORDS	DIV_ROUND_UP(_NSIG / _NSIG_BPW)
	 
	 typedef struct {
		unsigned long sig[_NSIG_WORDS];

just to avoid BUG_ON().

I agree that _NSIG == 126 or 64 needs more discussion. Although personally
I think this is the only choice in the long term, or we should change ABI
and break user-space completely.

And, just in case, the hack above doesn't kill SIG_128 completely.
Say, the task can block/unblock it.

Oleg.

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