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Date:   Tue, 16 Feb 2021 10:02:52 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Kees Cook <keescook@chromium.org>
Cc:     Chris Wilson <chris@chris-wilson.co.uk>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org,
        Andy Lutomirski <luto@amacapital.net>,
        Will Drewry <wad@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Lucas Stach <l.stach@pengutronix.de>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Cyrill Gorcunov <gorcunov@gmail.com>, stable@vger.kernel.org,
        Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without
 CHECKPOINT_RESTORE
Message-ID: <YCuKPOKlvSy/WiEZ@phenom.ffwll.local>
Mail-Followup-To: Kees Cook <keescook@chromium.org>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org,
        Andy Lutomirski <luto@amacapital.net>,
        Will Drewry <wad@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Airlie <airlied@gmail.com>,
        Lucas Stach <l.stach@pengutronix.de>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Cyrill Gorcunov <gorcunov@gmail.com>, stable@vger.kernel.org
References: <20210205163752.11932-1-chris@chris-wilson.co.uk>
 <20210205220012.1983-1-chris@chris-wilson.co.uk>
 <202102081411.73A442F17@keescook>
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On Mon, Feb 08, 2021 at 02:12:00PM -0800, Kees Cook wrote:
> On Fri, Feb 05, 2021 at 10:00:12PM +0000, Chris Wilson wrote:
> > Userspace has discovered the functionality offered by SYS_kcmp and has
> > started to depend upon it. In particular, Mesa uses SYS_kcmp for
> > os_same_file_description() in order to identify when two fd (e.g. device
> > or dmabuf) point to the same struct file. Since they depend on it for
> > core functionality, lift SYS_kcmp out of the non-default
> > CONFIG_CHECKPOINT_RESTORE into the selectable syscall category.
> > 
> > Rasmus Villemoes also pointed out that systemd uses SYS_kcmp to
> > deduplicate the per-service file descriptor store.
> > 
> > Note that some distributions such as Ubuntu are already enabling
> > CHECKPOINT_RESTORE in their configs and so, by extension, SYS_kcmp.
> > 
> > References: https://gitlab.freedesktop.org/drm/intel/-/issues/3046
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> 
> Thanks!
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>

Thanks for reviews&patch, I stuffed it into a topic branch and plan to
send it to Linus later this week.

Cheers, Daniel

> 
> -Kees
> 
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Andy Lutomirski <luto@amacapital.net>
> > Cc: Will Drewry <wad@chromium.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Dave Airlie <airlied@gmail.com>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > Cc: Lucas Stach <l.stach@pengutronix.de>
> > Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> > Cc: Cyrill Gorcunov <gorcunov@gmail.com>
> > Cc: stable@vger.kernel.org
> > Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> # DRM depends on kcmp
> > Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> # systemd uses kcmp
> > 
> > ---
> > v2:
> >   - Default n.
> >   - Borrrow help message from man kcmp.
> >   - Export get_epoll_tfile_raw_ptr() for CONFIG_KCMP
> > v3:
> >   - Select KCMP for CONFIG_DRM
> > ---
> >  drivers/gpu/drm/Kconfig                       |  3 +++
> >  fs/eventpoll.c                                |  4 ++--
> >  include/linux/eventpoll.h                     |  2 +-
> >  init/Kconfig                                  | 11 +++++++++++
> >  kernel/Makefile                               |  2 +-
> >  tools/testing/selftests/seccomp/seccomp_bpf.c |  2 +-
> >  6 files changed, 19 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> > index 0973f408d75f..af6c6d214d91 100644
> > --- a/drivers/gpu/drm/Kconfig
> > +++ b/drivers/gpu/drm/Kconfig
> > @@ -15,6 +15,9 @@ menuconfig DRM
> >  	select I2C_ALGOBIT
> >  	select DMA_SHARED_BUFFER
> >  	select SYNC_FILE
> > +# gallium uses SYS_kcmp for os_same_file_description() to de-duplicate
> > +# device and dmabuf fd. Let's make sure that is available for our userspace.
> > +	select KCMP
> >  	help
> >  	  Kernel-level support for the Direct Rendering Infrastructure (DRI)
> >  	  introduced in XFree86 4.0. If you say Y here, you need to select
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index a829af074eb5..3196474cbe24 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -979,7 +979,7 @@ static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
> >  	return epir;
> >  }
> >  
> > -#ifdef CONFIG_CHECKPOINT_RESTORE
> > +#ifdef CONFIG_KCMP
> >  static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long toff)
> >  {
> >  	struct rb_node *rbp;
> > @@ -1021,7 +1021,7 @@ struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd,
> >  
> >  	return file_raw;
> >  }
> > -#endif /* CONFIG_CHECKPOINT_RESTORE */
> > +#endif /* CONFIG_KCMP */
> >  
> >  /**
> >   * Adds a new entry to the tail of the list in a lockless way, i.e.
> > diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
> > index 0350393465d4..593322c946e6 100644
> > --- a/include/linux/eventpoll.h
> > +++ b/include/linux/eventpoll.h
> > @@ -18,7 +18,7 @@ struct file;
> >  
> >  #ifdef CONFIG_EPOLL
> >  
> > -#ifdef CONFIG_CHECKPOINT_RESTORE
> > +#ifdef CONFIG_KCMP
> >  struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsigned long toff);
> >  #endif
> >  
> > diff --git a/init/Kconfig b/init/Kconfig
> > index b77c60f8b963..9cc7436b2f73 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1194,6 +1194,7 @@ endif # NAMESPACES
> >  config CHECKPOINT_RESTORE
> >  	bool "Checkpoint/restore support"
> >  	select PROC_CHILDREN
> > +	select KCMP
> >  	default n
> >  	help
> >  	  Enables additional kernel features in a sake of checkpoint/restore.
> > @@ -1737,6 +1738,16 @@ config ARCH_HAS_MEMBARRIER_CALLBACKS
> >  config ARCH_HAS_MEMBARRIER_SYNC_CORE
> >  	bool
> >  
> > +config KCMP
> > +	bool "Enable kcmp() system call" if EXPERT
> > +	help
> > +	  Enable the kernel resource comparison system call. It provides
> > +	  user-space with the ability to compare two processes to see if they
> > +	  share a common resource, such as a file descriptor or even virtual
> > +	  memory space.
> > +
> > +	  If unsure, say N.
> > +
> >  config RSEQ
> >  	bool "Enable rseq() system call" if EXPERT
> >  	default y
> > diff --git a/kernel/Makefile b/kernel/Makefile
> > index aa7368c7eabf..320f1f3941b7 100644
> > --- a/kernel/Makefile
> > +++ b/kernel/Makefile
> > @@ -51,7 +51,7 @@ obj-y += livepatch/
> >  obj-y += dma/
> >  obj-y += entry/
> >  
> > -obj-$(CONFIG_CHECKPOINT_RESTORE) += kcmp.o
> > +obj-$(CONFIG_KCMP) += kcmp.o
> >  obj-$(CONFIG_FREEZER) += freezer.o
> >  obj-$(CONFIG_PROFILING) += profile.o
> >  obj-$(CONFIG_STACKTRACE) += stacktrace.o
> > diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > index 26c72f2b61b1..1b6c7d33c4ff 100644
> > --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> > +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > @@ -315,7 +315,7 @@ TEST(kcmp)
> >  	ret = __filecmp(getpid(), getpid(), 1, 1);
> >  	EXPECT_EQ(ret, 0);
> >  	if (ret != 0 && errno == ENOSYS)
> > -		SKIP(return, "Kernel does not support kcmp() (missing CONFIG_CHECKPOINT_RESTORE?)");
> > +		SKIP(return, "Kernel does not support kcmp() (missing CONFIG_KCMP?)");
> >  }
> >  
> >  TEST(mode_strict_support)
> > -- 
> > 2.20.1
> > 
> 
> -- 
> Kees Cook

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch