This series is based on torvalds/master.
The series is split up like so: - Patch 1 is a simple fixup which we should take in any case (even by itself). - Patches 2-6 add the feature, configurable selftest support, and docs.
Why not ...? ============
- Why not /proc/[pid]/userfaultfd? The proposed use case for this is for one process to open a userfaultfd which can intercept another process' page faults. This seems to me like exactly what CAP_SYS_PTRACE is for, though, so I think this use case can simply use a syscall without the powers CAP_SYS_PTRACE grants being "too much".
- Why not use a syscall? Access to syscalls is generally controlled by capabilities. We don't have a capability which is used for userfaultfd access without also granting more / other permissions as well, and adding a new capability was rejected [1].
- It's possible a LSM could be used to control access instead. I suspect adding a brand new one just for this would be rejected, but I think some existing ones like SELinux can be used to filter syscall access. Enabling SELinux for large production deployments which don't already use it is likely to be a huge undertaking though, and I don't think this use case by itself is enough to motivate that kind of architectural change.
Changelog =========
v3->v4: - Picked up an Acked-by on 5/5. - Updated cover letter to cover "why not ...". - Refactored userfaultfd_allowed() into userfaultfd_syscall_allowed(). [Peter] - Removed obsolete comment from a previous version. [Peter] - Refactored userfaultfd_open() in selftest. [Peter] - Reworded admin-guide documentation. [Mike, Peter] - Squashed 2 commits adding /dev/userfaultfd to selftest and making selftest configurable. [Peter] - Added "syscall" test modifier (the default behavior) to selftest. [Peter]
v2->v3: - Rebased onto linux-next/akpm-base, in order to be based on top of the run_vmtests.sh refactor which was merged previously. - Picked up some Reviewed-by's. - Fixed ioctl definition (_IO instead of _IOWR), and stopped using compat_ptr_ioctl since it is unneeded for ioctls which don't take a pointer. - Removed the "handle_kernel_faults" bool, simplifying the code. The result is logically equivalent, but simpler. - Fixed userfaultfd selftest so it returns KSFT_SKIP appropriately. - Reworded documentation per Shuah's feedback on v2. - Improved example usage for userfaultfd selftest.
v1->v2: - Add documentation update. - Test *both* userfaultfd(2) and /dev/userfaultfd via the selftest.
[1]: https://lore.kernel.org/lkml/686276b9-4530-2045-6bd8-170e5943abe4@schaufler-...
Axel Rasmussen (5): selftests: vm: add hugetlb_shared userfaultfd test to run_vmtests.sh userfaultfd: add /dev/userfaultfd for fine grained access control userfaultfd: selftests: modify selftest to use /dev/userfaultfd userfaultfd: update documentation to describe /dev/userfaultfd selftests: vm: add /dev/userfaultfd test cases to run_vmtests.sh
Documentation/admin-guide/mm/userfaultfd.rst | 41 +++++++++++- Documentation/admin-guide/sysctl/vm.rst | 3 + fs/userfaultfd.c | 69 ++++++++++++++++---- include/uapi/linux/userfaultfd.h | 4 ++ tools/testing/selftests/vm/run_vmtests.sh | 11 +++- tools/testing/selftests/vm/userfaultfd.c | 69 +++++++++++++++++--- 6 files changed, 169 insertions(+), 28 deletions(-)
-- 2.37.0.170.g444d1eabd0-goog