Re: [RFC/PATCH bpf-next] bpf: Fix d_path test after last fs update

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Aug 30, 2023 at 11:35:02AM +0200, Jiri Olsa wrote:
> Recent commit [1] broken d_path test, because now filp_close is not
> called directly from sys_close, but eventually later when the file
> is finally released.
> 
> I can't see any other solution than to hook filp_flush function and
> that also means we need to add it to btf_allowlist_d_path list, so
> it can use the d_path helper.
> 
> But it's probably not very stable because filp_flush is static so it
> could be potentially inlined.

looks like llvm makes it inlined (from CI)

  Error: #68/1 d_path/basic
  libbpf: prog 'prog_close': failed to find kernel BTF type ID of 'filp_flush': -3

jirka

> 
> Also if we'd keep the current filp_close hook and find a way how to 'wait'
> for it to be called so user space can go with checks, then it looks
> like d_path might not work properly when the task is no longer around.
> 
> thoughts?
> jirka
> 
> 
> [1] 021a160abf62 ("fs: use __fput_sync in close(2)")
> ---
>  kernel/trace/bpf_trace.c                        | 1 +
>  tools/testing/selftests/bpf/progs/test_d_path.c | 4 ++--
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index a7264b2c17ad..c829e24af246 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -941,6 +941,7 @@ BTF_ID(func, vfs_fallocate)
>  BTF_ID(func, dentry_open)
>  BTF_ID(func, vfs_getattr)
>  BTF_ID(func, filp_close)
> +BTF_ID(func, filp_flush)
>  BTF_SET_END(btf_allowlist_d_path)
>  
>  static bool bpf_d_path_allowed(const struct bpf_prog *prog)
> diff --git a/tools/testing/selftests/bpf/progs/test_d_path.c b/tools/testing/selftests/bpf/progs/test_d_path.c
> index 84e1f883f97b..3467d1b8098c 100644
> --- a/tools/testing/selftests/bpf/progs/test_d_path.c
> +++ b/tools/testing/selftests/bpf/progs/test_d_path.c
> @@ -40,8 +40,8 @@ int BPF_PROG(prog_stat, struct path *path, struct kstat *stat,
>  	return 0;
>  }
>  
> -SEC("fentry/filp_close")
> -int BPF_PROG(prog_close, struct file *file, void *id)
> +SEC("fentry/filp_flush")
> +int BPF_PROG(prog_close, struct file *file)
>  {
>  	pid_t pid = bpf_get_current_pid_tgid() >> 32;
>  	__u32 cnt = cnt_close;
> -- 
> 2.41.0
>