linux-kernel - Re: [PATCH] signal: Unexport sigsuspend()

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Message-ID: <20151117150035.GB5489@redhat.com>
Date:	Tue, 17 Nov 2015 16:00:35 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Richard Weinberger <richard@....at>
Cc:	linux-kernel@...r.kernel.org, vegard.nossum@...cle.com,
	akpm@...ux-foundation.org, amanieu@...il.com, dave@...olabs.net,
	qiaowei.ren@...el.com, luto@...capital.net, palmer@...belt.com,
	vdavydov@...allels.com
Subject: Re: [PATCH] signal: Unexport sigsuspend()

On 11/16, Richard Weinberger wrote:
>
> sigsuspend() is nowhere used except in signal.c itself,
> so we can mark it static do not pollute the global namespace.
>
> But this patch is more than a boring cleanup patch,
> it fixes a real issue on UserModeLinux.
> UML has a special console driver to display ttys using xterm,
> or other terminal emulators, on the host side.
> Vegard reported that sometimes UML is unable to spawn a xterm
> and he's facing the following warning:
> WARNING: CPU: 0 PID: 908 at include/linux/thread_info.h:128 sigsuspend+0xab/0xc0()
> It turned out that this warning makes absolutely no sense as
> the UML xterm code calls sigsuspend() on the host side, at least it tries.
> But as the kernel itself offers a sigsuspend() symbol the linker choose
> this one instead of the glibc wrapper. Interestingly this code used to
> work since ever but always blocked signals on the wrong side.
> Some recent kernel change made the WARN_ON() trigger and uncovered the bug.
>
> It is a wonderful example of how much works by chance on computers. :-)

You know, initially I didn't bother to read the changelog, I was going
to nack this change because git-grep reports that sigsuspend() is called
by arch/um/drivers/chan_user.c ;)

> Reported-and-tested-by: Vegard Nossum <vegard.nossum@...cle.com>
> Signed-off-by: Richard Weinberger <richard@....at>

Acked-by: Oleg Nesterov <oleg@...hat.com>

> ---
>  include/linux/signal.h | 1 -
>  kernel/signal.c        | 2 +-
>  2 files changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/linux/signal.h b/include/linux/signal.h
> index ab1e039..92557bb 100644
> --- a/include/linux/signal.h
> +++ b/include/linux/signal.h
> @@ -239,7 +239,6 @@ extern int sigprocmask(int, sigset_t *, sigset_t *);
>  extern void set_current_blocked(sigset_t *);
>  extern void __set_current_blocked(const sigset_t *);
>  extern int show_unhandled_signals;
> -extern int sigsuspend(sigset_t *);
>  
>  struct sigaction {
>  #ifndef __ARCH_HAS_IRIX_SIGACTION
> diff --git a/kernel/signal.c b/kernel/signal.c
> index c0b01fe..f3f1f7a 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -3503,7 +3503,7 @@ SYSCALL_DEFINE0(pause)
>  
>  #endif
>  
> -int sigsuspend(sigset_t *set)
> +static int sigsuspend(sigset_t *set)
>  {
>  	current->saved_sigmask = current->blocked;
>  	set_current_blocked(set);
> -- 
> 2.5.0
> 

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