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Subject: Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without
 CHECKPOINT_RESTORE
To:     Chris Wilson <chris@chris-wilson.co.uk>,
        linux-kernel@vger.kernel.org
Cc:     Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        intel-gfx@lists.freedesktop.org,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        dri-devel@lists.freedesktop.org,
        Andy Lutomirski <luto@amacapital.net>,
        Cyrill Gorcunov <gorcunov@gmail.com>, stable@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
References: <20210205163752.11932-1-chris@chris-wilson.co.uk>
 <20210205220012.1983-1-chris@chris-wilson.co.uk>
From:   Thomas Zimmermann <tzimmermann@suse.de>
Message-ID: <f3b09921-df08-b4fc-bd3b-1aaf583bc9a8@suse.de>
Date:   Mon, 15 Feb 2021 09:56:14 +0100
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From: Thomas Zimmermann <tzimmermann@suse.de>
To: Chris Wilson <chris@chris-wilson.co.uk>, linux-kernel@vger.kernel.org
Cc: Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
 Daniel Vetter <daniel.vetter@ffwll.ch>, intel-gfx@lists.freedesktop.org,
 Rasmus Villemoes <linux@rasmusvillemoes.dk>,
 dri-devel@lists.freedesktop.org, Andy Lutomirski <luto@amacapital.net>,
 Cyrill Gorcunov <gorcunov@gmail.com>, stable@vger.kernel.org,
 Andrew Morton <akpm@linux-foundation.org>
Message-ID: <f3b09921-df08-b4fc-bd3b-1aaf583bc9a8@suse.de>
Subject: Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without
 CHECKPOINT_RESTORE
References: <20210205163752.11932-1-chris@chris-wilson.co.uk>
 <20210205220012.1983-1-chris@chris-wilson.co.uk>
In-Reply-To: <20210205220012.1983-1-chris@chris-wilson.co.uk>

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Hi

Am 05.02.21 um 23:00 schrieb Chris Wilson:
> 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. devic=
e
> 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.
>=20
> Rasmus Villemoes also pointed out that systemd uses SYS_kcmp to
> deduplicate the per-service file descriptor store.

This helps a lot with transactional programming in userspace system=20
code. So FWIW

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>

>=20
> Note that some distributions such as Ubuntu are already enabling
> CHECKPOINT_RESTORE in their configs and so, by extension, SYS_kcmp.
>=20
> References: https://gitlab.freedesktop.org/drm/intel/-/issues/3046
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> 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 kc=
mp
>=20
> ---
> 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(-)
>=20
> 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 user=
space.
> +	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;
>   }
>  =20
> -#ifdef CONFIG_CHECKPOINT_RESTORE
> +#ifdef CONFIG_KCMP
>   static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsi=
gned long toff)
>   {
>   	struct rb_node *rbp;
> @@ -1021,7 +1021,7 @@ struct file *get_epoll_tfile_raw_ptr(struct file =
*file, int tfd,
>  =20
>   	return file_raw;
>   }
> -#endif /* CONFIG_CHECKPOINT_RESTORE */
> +#endif /* CONFIG_KCMP */
>  =20
>   /**
>    * 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;
>  =20
>   #ifdef CONFIG_EPOLL
>  =20
> -#ifdef CONFIG_CHECKPOINT_RESTORE
> +#ifdef CONFIG_KCMP
>   struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsi=
gned long toff);
>   #endif
>  =20
> 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=
=2E
> @@ -1737,6 +1738,16 @@ config ARCH_HAS_MEMBARRIER_CALLBACKS
>   config ARCH_HAS_MEMBARRIER_SYNC_CORE
>   	bool
>  =20
> +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 +=3D livepatch/
>   obj-y +=3D dma/
>   obj-y +=3D entry/
>  =20
> -obj-$(CONFIG_CHECKPOINT_RESTORE) +=3D kcmp.o
> +obj-$(CONFIG_KCMP) +=3D kcmp.o
>   obj-$(CONFIG_FREEZER) +=3D freezer.o
>   obj-$(CONFIG_PROFILING) +=3D profile.o
>   obj-$(CONFIG_STACKTRACE) +=3D stacktrace.o
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/test=
ing/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 =3D __filecmp(getpid(), getpid(), 1, 1);
>   	EXPECT_EQ(ret, 0);
>   	if (ret !=3D 0 && errno =3D=3D ENOSYS)
> -		SKIP(return, "Kernel does not support kcmp() (missing CONFIG_CHECKPO=
INT_RESTORE?)");
> +		SKIP(return, "Kernel does not support kcmp() (missing CONFIG_KCMP?)"=
);
>   }
>  =20
>   TEST(mode_strict_support)
>=20

--=20
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
(HRB 36809, AG N=C3=BCrnberg)
Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer


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