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Subject[PATCH v3] kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE
Date
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>
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
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