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Right now, a new process can't be forked in another time namespace if it shares mm with its parent. It is prohibited, because each time namespace has its own vvar page that is mapped into a process address space. When a process calls exec, it gets a new mm and so it could be "legal" to switch time namespace in that case. This was not implemented and now if we want to do this, we need to add another clone flag to not break backward compatibility. We don't have any user requests to switch times on exec except the vfork+exec combination, so there is no reason to add a new clone flag. As for vfork+exec, this should be safe to allow switching timens with the current clone flag. Right now, vfork (CLONE_VFORK | CLONE_VM) fails if a child is forked into another time namespace. With this change, vfork creates a new process in parent's timens, and the following exec does the actual switch to the target time namespace. Suggested-by: Florian Weimer <fweimer@redhat.com> Signed-off-by: Andrei Vagin <avagin@gmail.com> Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org> Signed-off-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/20220613060723.197407-1-avagin@gmail.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.