Re: [PATCH 0/5] linux-user: Support extended clone(CLONE_VM)

["Alex Bennée" <alex.bennee@linaro.org>]

Josh Kunz <jkz@google.com> writes:

> This patch series implements extended support for the `clone` system
> call. As best I can tell, any option combination including `CLONE_VM`
> should be supported with the addition of this patch series. The
> implementation is described in greater detail in the patches themselves.
>
> Testing:
>
>   * All targets built on x86_64.
>   * `make check` and `make check-tcg` are passing. Additional tests have
>     been added to `linux-test.c` to validate clone behavior.

Not for me - which probably means you don't have docker/podman setup or
cross compilers for all the various guests we have. See
tests/tcg/configure.sh

>
> Caveats:
>
>   * This series touches, but does not fix, several bits of code that are
>     racey (namely the sigact table and the fd trans table).
>   * `exit_group` does not perform the appropriate cleanup for non-thread
>     children created with `CLONE_VM`. CPUs for such children are never
>     cleaned up. The correct implementation of exit-group is non-trivial
>     (since it also needs to track/handle cleanup for threads in the
>     clone'd child process). Also, I don't fully understand the
>     interaction between QOM<->linux-user. My naive implementation based
>     on the current implementation `exit(2)` was regularly crashing. If
>     maintainers have suggestions for better ways to handle exit_group,
>     they would be greatly appreciated. 
>   * execve does not clean up the CPUs of clone'd children, for the same
>     reasons as `exit_group`.

Apart from "a more perfect emulation" is there a particular use case
served by the extra functionality? AIUI up until this point we've
basically supported glibc's use of clone() which has generally been
enough. I'm assuming you've come across stuff that needs this more fine
grained support?

>
> Josh Kunz (5):
>   linux-user: Refactor do_fork to use new `qemu_clone`
>   linux-user: Make fd_trans task-specific.
>   linux-user: Make sigact_table part of the task state.
>   linux-user: Support CLONE_VM and extended clone options
>   linux-user: Add PDEATHSIG test for clone process hierarchy.
>
>  linux-user/Makefile.objs            |   2 +-
>  linux-user/clone.c                  | 565 ++++++++++++++++++++++++++++
>  linux-user/clone.h                  |  27 ++
>  linux-user/fd-trans-tbl.c           |  13 +
>  linux-user/fd-trans-type.h          |  17 +
>  linux-user/fd-trans.c               |   3 -
>  linux-user/fd-trans.h               |  75 ++--
>  linux-user/main.c                   |   1 +
>  linux-user/qemu.h                   |  49 +++
>  linux-user/signal.c                 |  84 ++++-
>  linux-user/syscall.c                | 452 ++++++++++++----------
>  tests/tcg/multiarch/Makefile.target |   3 +
>  tests/tcg/multiarch/linux-test.c    | 227 ++++++++++-
>  13 files changed, 1264 insertions(+), 254 deletions(-)
>  create mode 100644 linux-user/clone.c
>  create mode 100644 linux-user/clone.h
>  create mode 100644 linux-user/fd-trans-tbl.c
>  create mode 100644 linux-user/fd-trans-type.h


-- 
Alex Bennée