[PATCH bpf-next] selftests/bpf: Make sure stashed kptr in local kptr is freed recursively

[PATCH bpf-next] selftests/bpf: Make sure stashed kptr in local kptr is freed recursively

[Amery Hung]

When dropping a local kptr, any kptr stashed into it is supposed to be
freed through bpf_obj_free_fields->__bpf_obj_drop_impl recursively. Add a
test to make sure it happens.

The test first stashes a referenced kptr to "struct task" into a local
kptr and gets the reference count of the task. Then, it drops the local
kptr and reads the reference count of the task again. Since
bpf_obj_free_fields and __bpf_obj_drop_impl will go through the local kptr
recursively during bpf_obj_drop, the dtor of the stashed task kptr should
eventually be called. The second reference count should be one less than
the first one.

Signed-off-by: Amery Hung <amery.hung@bytedance.com>
---
 .../selftests/bpf/progs/task_kfunc_success.c  | 30 ++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/task_kfunc_success.c b/tools/testing/selftests/bpf/progs/task_kfunc_success.c
index 3138bb689b0b..a55149015063 100644
--- a/tools/testing/selftests/bpf/progs/task_kfunc_success.c
+++ b/tools/testing/selftests/bpf/progs/task_kfunc_success.c
@@ -143,8 +143,9 @@ int BPF_PROG(test_task_acquire_leave_in_map, struct task_struct *task, u64 clone
 SEC("tp_btf/task_newtask")
 int BPF_PROG(test_task_xchg_release, struct task_struct *task, u64 clone_flags)
 {
-	struct task_struct *kptr;
+	struct task_struct *kptr, *acquired;
 	struct __tasks_kfunc_map_value *v, *local;
+	int refcnt, refcnt_after_drop;
 	long status;
 
 	if (!is_test_kfunc_task())
@@ -190,7 +191,34 @@ int BPF_PROG(test_task_xchg_release, struct task_struct *task, u64 clone_flags)
 		return 0;
 	}
 
+	/* Stash a copy into local kptr and check if it is released recursively */
+	acquired = bpf_task_acquire(kptr);
+	if (!acquired) {
+		err = 7;
+		bpf_obj_drop(local);
+		bpf_task_release(kptr);
+		return 0;
+	}
+	bpf_probe_read_kernel(&refcnt, sizeof(refcnt), &acquired->rcu_users);
+
+	acquired = bpf_kptr_xchg(&local->task, acquired);
+	if (acquired) {
+		err = 8;
+		bpf_obj_drop(local);
+		bpf_task_release(kptr);
+		bpf_task_release(acquired);
+		return 0;
+	}
+
 	bpf_obj_drop(local);
+
+	bpf_probe_read_kernel(&refcnt_after_drop, sizeof(refcnt_after_drop), &kptr->rcu_users);
+	if (refcnt != refcnt_after_drop + 1) {
+		err = 9;
+		bpf_task_release(kptr);
+		return 0;
+	}
+
 	bpf_task_release(kptr);
 
 	return 0;
-- 
2.20.1

Re: [PATCH bpf-next] selftests/bpf: Make sure stashed kptr in local kptr is freed recursively

[Martin KaFai Lau]

On 8/26/24 6:13 PM, Amery Hung wrote:
> When dropping a local kptr, any kptr stashed into it is supposed to be
> freed through bpf_obj_free_fields->__bpf_obj_drop_impl recursively. Add a
> test to make sure it happens.
> 
> The test first stashes a referenced kptr to "struct task" into a local
> kptr and gets the reference count of the task. Then, it drops the local
> kptr and reads the reference count of the task again. Since
> bpf_obj_free_fields and __bpf_obj_drop_impl will go through the local kptr
> recursively during bpf_obj_drop, the dtor of the stashed task kptr should
> eventually be called. The second reference count should be one less than
> the first one.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>

Re: [PATCH bpf-next] selftests/bpf: Make sure stashed kptr in local kptr is freed recursively

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Tue, 27 Aug 2024 01:13:01 +0000 you wrote:
> When dropping a local kptr, any kptr stashed into it is supposed to be
> freed through bpf_obj_free_fields->__bpf_obj_drop_impl recursively. Add a
> test to make sure it happens.
> 
> The test first stashes a referenced kptr to "struct task" into a local
> kptr and gets the reference count of the task. Then, it drops the local
> kptr and reads the reference count of the task again. Since
> bpf_obj_free_fields and __bpf_obj_drop_impl will go through the local kptr
> recursively during bpf_obj_drop, the dtor of the stashed task kptr should
> eventually be called. The second reference count should be one less than
> the first one.
> 
> [...]

Here is the summary with links:
  - [bpf-next] selftests/bpf: Make sure stashed kptr in local kptr is freed recursively
    https://git.kernel.org/bpf/bpf-next/c/bd0b4836a233

You are awesome, thank you!
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