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MAP_DMA ioctls might be called from various threads within a process, for example when using QEMU, the vCPU threads are often generating these calls and we therefore take a reference to that vCPU task. However, QEMU also supports vCPU hotplug on some machines and the task that called MAP_DMA may have exited by the time UNMAP_DMA is called, resulting in the mm_struct pointer being NULL and thus a failure to match against the existing mapping. To resolve this, we instead take a reference to the thread group_leader, which has the same mm_struct and resource limits, but is less likely exit, at least in the QEMU case. A difficulty here is guaranteeing that the capabilities of the group_leader match that of the calling thread, which we resolve by tracking CAP_IPC_LOCK at the time of calling rather than at an indeterminate time in the future. Potentially this also results in better efficiency as this is now recorded once per MAP_DMA ioctl. Reported-by: Xu Yandong <xuyandong2@huawei.com> Signed-off-by: Alex Williamson <alex.williamson@redhat.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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.