netdev - Re: [PATCH bpf-next] selftests/bpf: Fix stat probe in d

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Date:   Wed, 16 Sep 2020 18:45:31 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix stat probe in d_path test

On Wed, Sep 16, 2020 at 01:24:16PM +0200, Jiri Olsa wrote:
> Some kernels builds might inline vfs_getattr call within fstat
> syscall code path, so fentry/vfs_getattr trampoline is not called.
> 
> Alexei suggested [1] we should use security_inode_getattr instead,
> because it's less likely to get inlined.
> 
> Adding security_inode_getattr to the d_path allowed list and
> switching the stat trampoline to security_inode_getattr.
> 
> Adding flags that indicate trampolines were called and failing
> the test if any of them got missed, so it's easier to identify
> the issue next time.
> 
> [1] https://lore.kernel.org/bpf/CAADnVQJ0FchoPqNWm+dEppyij-MOvvEG_trEfyrHdabtcEuZGg@mail.gmail.com/
> Fixes: e4d1af4b16f8 ("selftests/bpf: Add test for d_path helper")
> Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> ---
>  kernel/trace/bpf_trace.c                        | 1 +
>  tools/testing/selftests/bpf/prog_tests/d_path.c | 6 ++++++
>  tools/testing/selftests/bpf/progs/test_d_path.c | 9 ++++++++-
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index b2a5380eb187..1001c053ebb3 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1122,6 +1122,7 @@ BTF_ID(func, vfs_truncate)
>  BTF_ID(func, vfs_fallocate)
>  BTF_ID(func, dentry_open)
>  BTF_ID(func, vfs_getattr)
> +BTF_ID(func, security_inode_getattr)
>  BTF_ID(func, filp_close)
>  BTF_SET_END(btf_allowlist_d_path)

I think it's concealing the problem instead of fixing it.
bpf is difficult to use for many reasons. Let's not make it harder.
The users will have a very hard time debugging why vfs_getattr bpf probe
is not called in all cases.
Let's replace:
vfs_truncate -> security_path_truncate
vfs_fallocate -> security_file_permission
vfs_getattr -> security_inode_getattr

For dentry_open also add security_file_open.
dentry_open and filp_close are in its own files,
so unlikely to be inlined.
Ideally resolve_btfids would parse dwarf info and check
whether any of the funcs in allowlist were inlined.
That would be more reliable, but not pretty to drag libdw
dependency into resolve_btfids.

>  
> diff --git a/tools/testing/selftests/bpf/prog_tests/d_path.c b/tools/testing/selftests/bpf/prog_tests/d_path.c
> index fc12e0d445ff..f507f1a6fa3a 100644
> --- a/tools/testing/selftests/bpf/prog_tests/d_path.c
> +++ b/tools/testing/selftests/bpf/prog_tests/d_path.c
> @@ -120,6 +120,12 @@ void test_d_path(void)
>  	if (err < 0)
>  		goto cleanup;
>  
> +	if (CHECK(!bss->called_stat || !bss->called_close,

+1 to KP's comment.

> +		  "check",
> +		  "failed to call trampolines called_stat %d, bss->called_close %d\n",
> +		   bss->called_stat, bss->called_close))
> +		goto cleanup;