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 2-5 add the feature, configurable selftest support, and docs.
Why not ...? ============
- Why not /proc/[pid]/userfaultfd? Two main points (additional discussion [1]):
- /proc/[pid]/* files are all owned by the user/group of the process, and they don't really support chmod/chown. So, without extending procfs it doesn't solve the problem this series is trying to solve.
- The main argument *for* this was to support creating UFFDs for remote processes. But, that use case clearly calls for CAP_SYS_PTRACE, so to support this we could just use the UFFD syscall as-is.
- 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 [2].
- It's possible a LSM could be used to control access instead, but I have some concerns. I don't think this approach would be as easy to use, particularly if we were to try to solve this with something heavyweight like SELinux. Maybe we could pursue adding a new LSM specifically for this user case, but it may be too narrow of a case to justify that.
Changelog =========
v5->v6: - Modified selftest to exit with KSFT_SKIP *only* when features are unsupported, exiting with 1 in other error cases. [Mike] - Improved wording in two spots in the documentation. [Mike] - Picked up some Acked-by's.
v4->v5: - Call userfaultfd_syscall_allowed() directly in the syscall, so we don't have to plumb a flag into new_userfaultfd(). [Nadav] - Refactored run_vmtests.sh to loop over UFFD test mods. [Nadav] - Reworded cover letter. - Picked up some Acked-by's.
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://patchwork.kernel.org/project/linux-mm/cover/20220719195628.3415852-1... [2]: 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 | 73 ++++++++++++++----- include/uapi/linux/userfaultfd.h | 4 ++ tools/testing/selftests/vm/run_vmtests.sh | 15 ++-- tools/testing/selftests/vm/userfaultfd.c | 76 +++++++++++++++++--- 6 files changed, 178 insertions(+), 34 deletions(-)
-- 2.37.1.595.g718a3a8f04-goog