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    SubjectRe: [PATCH] kernel: Expose SYS_kcmp by default
    On 2021-02-08 2:34 p.m., Daniel Vetter wrote:
    > On Mon, Feb 8, 2021 at 12:49 PM Michel Dänzer <> wrote:
    >> On 2021-02-05 9:53 p.m., Daniel Vetter wrote:
    >>> On Fri, Feb 5, 2021 at 7:37 PM Kees Cook <> wrote:
    >>>> On Fri, Feb 05, 2021 at 04:37:52PM +0000, Chris Wilson wrote:
    >>>>> 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.
    >>>>> Signed-off-by: Chris Wilson <>
    >>>>> Cc: Kees Cook <>
    >>>>> Cc: Andy Lutomirski <>
    >>>>> Cc: Will Drewry <>
    >>>>> Cc: Andrew Morton <>
    >>>>> Cc: Dave Airlie <>
    >>>>> Cc: Daniel Vetter <>
    >>>>> Cc: Lucas Stach <>
    >>>>> ---
    >>>>> init/Kconfig | 11 +++++++++++
    >>>>> kernel/Makefile | 2 +-
    >>>>> tools/testing/selftests/seccomp/seccomp_bpf.c | 2 +-
    >>>>> 3 files changed, 13 insertions(+), 2 deletions(-)
    >>>>> diff --git a/init/Kconfig b/init/Kconfig
    >>>>> index b77c60f8b963..f62fca13ac5b 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
    >>>>> bool
    >>>>> +config KCMP
    >>>>> + bool "Enable kcmp() system call" if EXPERT
    >>>>> + default y
    >>>> I would expect this to be not default-y, especially if
    >>>> CHECKPOINT_RESTORE does a "select" on it.
    >>>> This is a really powerful syscall, but it is bounded by ptrace access
    >>>> controls, and uses pointer address obfuscation, so it may be okay to
    >>>> expose this. As it is, at least Ubuntu already has
    >>>> CONFIG_CHECKPOINT_RESTORE, so really, there's probably not much
    >>>> difference on exposure.
    >>>> So, if you drop the "default y", I'm fine with this.
    >>> It was maybe stupid, but our userspace started relying on fd
    >>> comaprison through sys_kcomp. So for better or worse, if you want to
    >>> run the mesa3d gl/vk stacks, you need this.
    >> That's overstating things somewhat. The vast majority of applications
    >> will work fine regardless (as they did before Mesa started using this
    >> functionality). Only some special ones will run into issues, because the
    >> user-space drivers incorrectly assume two file descriptors reference
    >> different descriptions.
    >>> Was maybe not the brighest ideas, but since enough distros had this
    >>> enabled by defaults,
    >> Right, that (and the above) is why I considered it fair game to use.
    >> What should I have done instead? (TBH I was surprised that this
    >> functionality isn't generally available)
    > Yeah that one is fine, but I thought we've discussed (irc or
    > something) more uses for de-duping dma-buf and stuff like that. But
    > quick grep says that hasn't landed yet, so I got a bit confused (or
    > just dreamt). Looking at this again I'm kinda surprised the drmfd
    > de-duping blows up on normal linux distros, but I guess it can all
    > happen.

    One example: GEM handle name-spaces are per file description. If
    user-space incorrectly assumes two DRM fds are independent, when they
    actually reference the same file description, closing a GEM handle with
    one file descriptor will make it unusable with the other file descriptor
    as well.

    >>> Ofc we can leave the default n, but the select if CONFIG_DRM is
    >>> unfortunately needed I think.
    >> Per above, not sure this is really true.
    > We seem to be going boom on linux distros now, maybe userspace got
    > more creative in abusing stuff?

    I don't know what you're referring to. I've only seen maybe two or three
    reports from people who didn't enable CHECKPOINT_RESTORE in their
    self-built kernels.

    > The entire thing is small enough that imo we don't really have to care,
    > e.g. we also unconditionally select dma-buf, despite that on most
    > systems there's only 1 gpu, and you're never going to end up with a
    > buffer sharing case that needs any of that code (aside from the
    > "here's an fd" part).
    > But I guess we can limit to just KCMP_FILE like you suggest in another
    > reply. Just feels a bit like overkill.

    Making KCMP_FILE gated by DRM makes as little sense to me as by

    Earthling Michel Dänzer |
    Libre software enthusiast | Mesa and X developer

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