Re: [PATCH bpf-next] selftests/bpf: fix potential premature unload in bpf_testmod

[Yonghong Song <yonghong.song@linux.dev>]

On 1/9/24 8:43 AM, Artem Savkov wrote:
> It is possible for bpf_kfunc_call_test_release() to be called from
> bpf_map_free_deferred() when bpf_testmod is already unloaded and
> perf_test_stuct.cnt which it tries to decrease is no longer in memory.
> This patch tries to fix the issue by waiting for all references to be
> dropped in bpf_testmod_exit().
>
> The issue can be triggered by running 'test_progs -t map_kptr' in 6.5,
> but is obscured in 6.6 by d119357d07435 ("rcu-tasks: Treat only
> synchronous grace periods urgently").
>
> Fixes: 65eb006d85a2a ("bpf: Move kernel test kfuncs to bpf_testmod")

Please add your Signed-off-by tag.

I think the root cause is that bpf_kfunc_call_test_acquire() kfunc
is defined in bpf_testmod and the kfunc returns some data in bpf_testmod.
But the release function bpf_kfunc_call_test_release() is in the kernel.
The release func tries to access some data in bpf_testmod which might
have been unloaded. The prog_test_ref_kfunc is defined in the kernel, so
no bpf_testmod btf reference is hold so bpf_testmod can be unloaded before
bpf_kfunc_call_test_release().
As you mentioned, we won't have this issue if bpf_kfunc_call_test_acquire()
is also in the kernel.

I think putting bpf_kfunc_call_test_acquire() in bpf_testmod and
bpf_kfunc_call_test_release() in kernel is not a good idea and confusing.
But since this is only for tests, I guess we can live with that. With that,

Acked-by: Yonghong Song <yonghong.song@linux.dev>

> ---
>   tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> index 91907b321f913..63f0dbd016703 100644
> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> @@ -2,6 +2,7 @@
>   /* Copyright (c) 2020 Facebook */
>   #include <linux/btf.h>
>   #include <linux/btf_ids.h>
> +#include <linux/delay.h>
>   #include <linux/error-injection.h>
>   #include <linux/init.h>
>   #include <linux/module.h>
> @@ -544,6 +545,9 @@ static int bpf_testmod_init(void)
>   
>   static void bpf_testmod_exit(void)
>   {
> +	while (refcount_read(&prog_test_struct.cnt) > 1)
> +		msleep(20);
> +
>   	return sysfs_remove_bin_file(kernel_kobj, &bin_attr_bpf_testmod_file);
>   }
>