[PATCH v1 0/4] lib: introduce copy_struct_from_user() helper

This series was split off from the openat2(2) syscall discussion[1].
However, the copy_struct_to_user() helper has been dropped, because
after some discussion it appears that there is no really obvious
semantics for how copy_struct_to_user() should work on mixed-vintages
(for instance, whether [2] is the correct semantics for all syscalls).

A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases).

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[3]). This series
implements the helper and ports several syscalls to use it.

[1]: https://lore.kernel.org/lkml/20190904201933.10736-1-cyphar@cyphar.com/ (local)

[2]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
     robustify sched_read_attr() ABI logic and code")

[3]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
     similar checks to copy_struct_from_user() while rt_sigprocmask(2)
     always rejects differently-sized struct arguments.

Aleksa Sarai (4):
  lib: introduce copy_struct_from_user() helper
  clone3: switch to copy_struct_from_user()
  sched_setattr: switch to copy_struct_from_user()
  perf_event_open: switch to copy_struct_from_user()

 include/linux/uaccess.h    |  4 +++
 include/uapi/linux/sched.h |  2 ++
 kernel/events/core.c       | 47 +++++-------------------
 kernel/fork.c              | 34 ++++--------------
 kernel/sched/core.c        | 43 ++++------------------
 lib/Makefile               |  2 +-
 lib/strnlen_user.c         | 52 +++++++++++++++++++++++++++
 lib/struct_user.c          | 73 ++++++++++++++++++++++++++++++++++++++
 8 files changed, 155 insertions(+), 102 deletions(-)
 create mode 100644 lib/struct_user.c

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2.23.0