Re: [PATCH bpf-next v3 0/2] Update KF_RCU_PROTECTED

[Andrea Righi <arighi@nvidia.com>]

On Wed, Sep 17, 2025 at 03:27:53AM +0000, Kumar Kartikeya Dwivedi wrote:
> Currently, KF_RCU_PROTECTED only applies to iterator APIs and that too
> in a convoluted fashion: the presence of this flag on the kfunc is used
> to set MEM_RCU in iterator type, and the lack of RCU protection results
> in an error only later, once next() or destroy() methods are invoked on
> the iterator. While there is no bug, this is certainly a bit unintuitive,
> and makes the enforcement of the flag iterator specific.
> 
> In the interest of making this flag useful for other upcoming kfuncs,
> e.g. scx_bpf_cpu_curr() [0][1], add enforcement for invoking the kfunc
> in an RCU critical section in general.
> 
> In addition to this, the aforementioned kfunc also needs to return an
> RCU protected pointer, which currently has no generic kfunc flag or
> annotation. Add such a flag as well while we are at it.
> 
>   [0]: https://lore.kernel.org/all/20250903212311.369697-3-christian.loehle@arm.com
>   [1]: https://lore.kernel.org/all/20250909195709.92669-1-arighi@nvidia.com

Everything looks good from a sched_ext perspective.

I've also tested this with the new scx_bpf_cpu_curr() kfunc, marked as
KF_RCU_PROTECTED, and everything seems to work as expected.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> 
> Changelog:
> ----------
> v2 -> v3
> v2: https://lore.kernel.org/bpf/20250917032014.4060112-1-memxor@gmail.com
> 
>  * Add back lost hunk reworking documentation for KF_RCU_PROTECTED.
> 
> v1 -> v2
> v1: https://lore.kernel.org/bpf/20250915024731.1494251-1-memxor@gmail.com
> 
>  * Drop KF_RET_RCU and fold change into KF_RCU_PROTECTED. (Andrea, Alexei)
>  * Update tests for non-struct pointer return values with KF_RCU_PROTECTED.
> 
> Kumar Kartikeya Dwivedi (2):
>   bpf: Enforce RCU protection for KF_RCU_PROTECTED
>   selftests/bpf: Add tests for KF_RCU_PROTECTED
> 
>  Documentation/bpf/kfuncs.rst                  | 19 +++++++-
>  kernel/bpf/verifier.c                         | 10 ++++
>  .../selftests/bpf/progs/cgroup_read_xattr.c   |  2 +-
>  .../selftests/bpf/progs/iters_task_failure.c  |  4 +-
>  .../selftests/bpf/progs/iters_testmod.c       | 46 +++++++++++++++++++
>  .../selftests/bpf/test_kmods/bpf_testmod.c    | 12 +++++
>  .../bpf/test_kmods/bpf_testmod_kfunc.h        |  2 +
>  7 files changed, 91 insertions(+), 4 deletions(-)
> 
> 
> base-commit: b13448dd64e27752fad252cec7da1a50ab9f0b6f
> -- 
> 2.51.0
>