Re: [PATCH bpf v3 0/2] bpf: Reject bare pointer for __arg_trusted arg

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Tue, Oct 06, 2026 at 04:06 PM Yiyang Chen <chenyy23@mails.tsinghua.edu.cn> wrote:
> Reject the bare pointer while preserving referenced, trusted, and
> RCU-protected arguments. The latter is used by sched-ext programs that pass
> the result of an RCU-protected kfunc through trusted-and-nullable global
> subprogram arguments.

Which programs?
veristat-scx failed on v2. The only one I see in scx repo is
cake_wake_place() in scx_cake. It takes the result of scx_bpf_cpu_curr()
as __arg_trusted __arg_nullable.
That arg was added two weeks ago. The last scx release doesn't have it.
On kernels without the kfunc cake_wake_place() already reads curr
by itself. Pls send a fix to scx instead.

rcu_ptr_ is not trusted. The refcount can be zero.
The callee sees trusted_ptr_ and can pass it to a kfunc that is not KF_RCU.
bpf_cpumask_acquire() does plain refcount_inc().
With rcu_ptr_bpf_cpumask loaded from a map it increments from zero
and the prog holds a reference to a cpumask that is freed after GP.
Commit e2b3c4ff5d18 says
"only PTR_TRUSTED flavor of PTR_TO_BTF_ID is supported".
v2 was right to reject it. Add the above to the commit log.

> This series targets bpf, which uses separate global-subprogram and kfunc
> argument checkers.

That's not an answer to Amery:
https://lore.kernel.org/bpf/CAMB2axNnY5wxkTrd3_tHyXBzXYyjJUkqRb1uNbUquwTJ_FLVMg@mail.gmail.com/
The loop in btf_check_func_arg_match() is gone in bpf-next. See
commit 668a51c4ed4b ("bpf: Build argument prototypes for subprog calls").
This patch will conflict when bpf is merged into bpf-next and the fix
has to be written again in check_func_arg().
The bug is there since 6.9 and the fix rejects progs that load today.
Pls target bpf-next and do what Amery suggested.

Also 12 chars of sha in the Fixes tag.

pw-bot: cr