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SubjectRe: [PATCH v2] kernel: Expose SYS_kcmp by default
On Fri, Feb 05, 2021 at 09:16:01PM +0000, Chris Wilson wrote:
> The subject should of course be changed, as it is no longer being
> enabled by default.

"default n" is redundant. I thought Daniel said CONFIG_DRM needed to
"select" it too, though? Otherwise, yeah, this looks good. Was the
export due to the 0-day bot failure reports?

-Kees

>
> Something like
>
> kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE
>
> Quoting Chris Wilson (2021-02-05 21:06:10)
> > 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.
> >
> > 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>
> > Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> # DRM depends on SYS_kcmp
> >
> > ---
> > v2:
> > - Default n.
> > - Borrrow help message from man kcmp.
> > - Export get_epoll_tfile_raw_ptr() for CONFIG_KCMP
> > ---
> > fs/eventpoll.c | 4 ++--
> > include/linux/eventpoll.h | 2 +-
> > init/Kconfig | 12 ++++++++++++
> > kernel/Makefile | 2 +-
> > tools/testing/selftests/seccomp/seccomp_bpf.c | 2 +-
> > 5 files changed, 17 insertions(+), 5 deletions(-)
> >
> > 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..1b75141bc18b 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 +1738,17 @@ config ARCH_HAS_MEMBARRIER_CALLBACKS
> > config ARCH_HAS_MEMBARRIER_SYNC_CORE
> > bool
> >
> > +config KCMP
> > + bool "Enable kcmp() system call" if EXPERT
> > + default n
> > + 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

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