Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

Peter Zijlstra
2023-12-08T20:35:50Z

On Fri, Dec 08, 2023 at 11:40:27AM -0800, Alexei Starovoitov wrote:

> typedef void (*btf_dtor_kfunc_t)(void *);
> btf_dtor_kfunc_t dtor;
> but the bpf_cgroup_release takes 'struct cgroup*'.
> From kcfi pov void * == struct cgroup * ?
> Do we need to change it to 'void *cgrp' ?

Yes, doing that naively like the below, gets me lovely things like:

validate_case:FAIL:expect_msg unexpected error: -22
VERIFIER LOG:
=============
=============
EXPECTED MSG: 'Possibly NULL pointer passed to trusted arg0'
#48/7 cgrp_kfunc/cgrp_kfunc_acquire_untrusted:FAIL
run_subtest:PASS:obj_open_mem 0 nsec
libbpf: extern (func ksym) 'bpf_cgroup_release': func_proto [148] incompatible with vmlinux [125610]
libbpf: failed to load object 'cgrp_kfunc_failure'

But let me try rebuilding everything..

---
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b3be5742d6f1..078b207af7f0 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2145,10 +2145,11 @@ __bpf_kfunc struct task_struct *bpf_task_acquire(struct task_struct *p)
* bpf_task_release - Release the reference acquired on a task.
* @p: The task on which a reference is being released.
*/
-__bpf_kfunc void bpf_task_release(struct task_struct *p)
+__bpf_kfunc void bpf_task_release(void *p)
{
put_task_struct_rcu_user(p);
}
+EXPORT_SYMBOL_GPL(bpf_task_release);

#ifdef CONFIG_CGROUPS
/**
@@ -2169,10 +2170,11 @@ __bpf_kfunc struct cgroup *bpf_cgroup_acquire(struct cgroup *cgrp)
* drops to 0.
* @cgrp: The cgroup on which a reference is being released.
*/
-__bpf_kfunc void bpf_cgroup_release(struct cgroup *cgrp)
+__bpf_kfunc void bpf_cgroup_release(void *cgrp)
{
cgroup_put(cgrp);
}
+EXPORT_SYMBOL_GPL(bpf_cgroup_release);

/**
* bpf_cgroup_ancestor - Perform a lookup on an entry in a cgroup's ancestor