Re: [PATCH] clone: fix CLONE_PIDFD support

From: "Dmitry V. Levin" <ldv@altlinux.org>
To: Christian Brauner <christian@brauner.io>
Cc: Anatoly Pugachev <matorola@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] clone: fix CLONE_PIDFD support
Date: Sun, 14 Jul 2019 19:14:49 +0300	[thread overview]
Message-ID: <20190714161449.GA10389@altlinux.org> (raw)
In-Reply-To: <20190714142304.3uihy4vepmxgdqha@brauner.io>

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On Sun, Jul 14, 2019 at 04:23:05PM +0200, Christian Brauner wrote:
> On Sun, Jul 14, 2019 at 05:10:08PM +0300, Dmitry V. Levin wrote:
> > On Sun, Jul 14, 2019 at 02:17:25PM +0200, Christian Brauner wrote:
> > > On Sun, Jul 14, 2019 at 03:02:06PM +0300, Dmitry V. Levin wrote:
> > > > The introduction of clone3 syscall accidentally broke CLONE_PIDFD
> > > > support in traditional clone syscall on compat x86 and those
> > > > architectures that use do_fork to implement clone syscall.
> > > > 
> > > > This bug was found by strace test suite.
> > > > 
> > > > Link: https://strace.io/logs/strace/2019-07-12
> > > > Fixes: 7f192e3cd316 ("fork: add clone3")
> > > > Bisected-and-tested-by: Anatoly Pugachev <matorola@gmail.com>
> > > > Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
> > > 
> > > Good catch! Thank you Dmitry.
> > > 
> > > One change request below.
> > > 
> > > > ---
> > > >  arch/x86/ia32/sys_ia32.c | 1 +
> > > >  kernel/fork.c            | 1 +
> > > >  2 files changed, 2 insertions(+)
> > > > 
> > > > diff --git a/arch/x86/ia32/sys_ia32.c b/arch/x86/ia32/sys_ia32.c
> > > > index 64a6c952091e..98754baf411a 100644
> > > > --- a/arch/x86/ia32/sys_ia32.c
> > > > +++ b/arch/x86/ia32/sys_ia32.c
> > > > @@ -239,6 +239,7 @@ COMPAT_SYSCALL_DEFINE5(x86_clone, unsigned long, clone_flags,
> > > >  {
> > > >  	struct kernel_clone_args args = {
> > > >  		.flags		= (clone_flags & ~CSIGNAL),
> > > > +		.pidfd		= parent_tidptr,
> > > >  		.child_tid	= child_tidptr,
> > > >  		.parent_tid	= parent_tidptr,
> > > >  		.exit_signal	= (clone_flags & CSIGNAL),
> > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > index 8f3e2d97d771..2c3cbad807b6 100644
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -2417,6 +2417,7 @@ long do_fork(unsigned long clone_flags,
> > > >  {
> > > >  	struct kernel_clone_args args = {
> > > >  		.flags		= (clone_flags & ~CSIGNAL),
> > > > +		.pidfd		= parent_tidptr,
> > > >  		.child_tid	= child_tidptr,
> > > >  		.parent_tid	= parent_tidptr,
> > > >  		.exit_signal	= (clone_flags & CSIGNAL),
> > > > -- 
> > > 
> > > Both of these legacy clone helpers need to make CLONE_PIDFD and
> > > CLONE_PARENT_SETTID incompatible, i.e. could you please add a helper to
> > > kernel/fork.c:
> > > 
> > > bool legacy_clone_args_valid(const struct kernel_clone_args *kargs)
> > > {
> > > 	/* clone(CLONE_PIDFD) uses parent_tidptr to return a pidfd */
> > > 	if ((kargs->flags & CLONE_PIDFD) && (kargs->flags & CLONE_PARENT_SETTID))
> > > 		return false;
> > > }
> > > 
> > > and export it and use it in ia32 too?
> > 
> > copy_process already performs the check, isn't this enough?
> 
> No it doesn't anymore. clone3() allows CLONE_PIDFD + CLONE_PARENT_SETTID
> since struct clone_args has a dedicated pidfd return argument.

Indeed, I must've missed it, thanks.

OK, I'll send a new fix with legacy_clone_args_valid.

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ldv

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