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    Subject[PATCH 5.10 620/663] kcmp: Support selection of SYS_kcmp without CHECKPOINT_RESTORE
    Date
    From: Chris Wilson <chris@chris-wilson.co.uk>

    commit bfe3911a91047557eb0e620f95a370aee6a248c7 upstream.

    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.

    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
    Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
    Reviewed-by: Kees Cook <keescook@chromium.org>
    Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Link: https://patchwork.freedesktop.org/patch/msgid/20210205220012.1983-1-chris@chris-wilson.co.uk
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    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(-)

    --- 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
    --- a/fs/eventpoll.c
    +++ b/fs/eventpoll.c
    @@ -1062,7 +1062,7 @@ static struct epitem *ep_find(struct eve
    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;
    @@ -1104,7 +1104,7 @@ struct file *get_epoll_tfile_raw_ptr(str

    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.
    --- 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

    --- 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
    --- a/kernel/Makefile
    +++ b/kernel/Makefile
    @@ -48,7 +48,7 @@ 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
    --- 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)

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