Re: [PATCH v3] kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE

Thomas Zimmermann <tzimmermann@xxxxxxx> · Mon, 15 Feb 2021 09:56:14 +0100

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. 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.

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

Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>


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@xxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx> # DRM depends on kcmp
Acked-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> # 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)


--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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