Re: [PATCH bpf-next] selftests/bpf: Check trampoline execution in d_path test

[Jiri Olsa <jolsa@redhat.com>]

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@kernel.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