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

[Christian Brauner]

On Wed, Sep 25, 2019 at 06:59:11PM +0200, Aleksa Sarai wrote:
> 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@xxxxxxxxxx/
> 
> [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.

Thank for splitting this out! :)
I should be able to review this tomorrow.

Christian