netdev - Re: [PATCH bpf-next] selftests/bpf: Check trampoline execution in d

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Date:   Fri, 11 Sep 2020 15:15:58 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <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: Check trampoline execution in
 d_path test

On Thu, Sep 10, 2020 at 05:46:21PM -0700, Alexei Starovoitov wrote:
> On Thu, Sep 10, 2020 at 5:22 AM Jiri Olsa <jolsa@...nel.org> wrote:
> >
> > Some kernels builds might inline vfs_getattr call within
> > fstat syscall code path, so fentry/vfs_getattr trampoline
> > is not called.
> >
> > I'm not sure how to handle this in some generic way other
> > than use some other function, but that might get inlined at
> > some point as well.
> 
> It's great that we had the test and it failed.
> Doing the test skipping will only hide the problem.
> Please don't do it here and in the future.
> Instead let's figure out the real solution.
> Assuming that vfs_getattr was added to btf_allowlist_d_path
> for a reason we have to make this introspection place
> reliable regardless of compiler inlining decisions.
> We can mark it as 'noinline', but that's undesirable.
> I suggest we remove it from the allowlist and replace it with
> security_inode_getattr.
> I think that is a better long term fix.

in my case vfs_getattr got inlined in vfs_statx_fd and both
of them are defined in fs/stat.c 

so the idea is that inlining will not happen if the function
is defined in another object? or less likely..?

we should be safe when it's called from module

> While at it I would apply the same critical thinking to other
> functions in the allowlist. They might suffer the same issue.
> So s/vfs_truncate/security_path_truncate/ and so on?
> Things won't work when CONFIG_SECURITY is off, but that is a rare kernel config?
> Or add both security_* and vfs_* variants and switch tests to use security_* ?
> but it feels fragile to allow inline-able funcs in allowlist.

hm, what's the difference between vfs_getattr and security_inode_getattr
in this regard? I'd expect compiler could inline it same way as for vfs_getattr

jirka