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From: Song Liu <song@kernel.org>
Date: Mon, 27 Nov 2023 09:16:32 -0800
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Subject: Re: [PATCH v13 bpf-next 5/6] selftests/bpf: Add tests for filesystem kfuncs
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, LSM List <linux-security-module@vger.kernel.org>, 
	Linux-Fsdevel <linux-fsdevel@vger.kernel.org>, fsverity@lists.linux.dev, 
	Eric Biggers <ebiggers@kernel.org>, Alexei Starovoitov <ast@kernel.org>, 
	Daniel Borkmann <daniel@iogearbox.net>, Andrii Nakryiko <andrii@kernel.org>, 
	Martin KaFai Lau <martin.lau@linux.dev>, Christian Brauner <brauner@kernel.org>, 
	Alexander Viro <viro@zeniv.linux.org.uk>, Casey Schaufler <casey@schaufler-ca.com>, 
	Amir Goldstein <amir73il@gmail.com>, KP Singh <kpsingh@kernel.org>, 
	Roberto Sassu <roberto.sassu@huawei.com>
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On Sun, Nov 26, 2023 at 6:09=E2=80=AFPM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Thu, Nov 23, 2023 at 3:40=E2=80=AFPM Song Liu <song@kernel.org> wrote:
> >
> > +static const char expected_value[] =3D "hello";
> > +char value[32];
> > +
> > +SEC("lsm.s/file_open")
> > +int BPF_PROG(test_file_open, struct file *f)
> > +{
> > +       struct bpf_dynptr value_ptr;
> > +       __u32 pid;
> > +       int ret;
> > +
> > +       pid =3D bpf_get_current_pid_tgid() >> 32;
> > +       if (pid !=3D monitored_pid)
> > +               return 0;
> > +
> > +       bpf_dynptr_from_mem(value, sizeof(value), 0, &value_ptr);
> > +
> > +       ret =3D bpf_get_file_xattr(f, "user.kfuncs", &value_ptr);
> > +       if (ret !=3D sizeof(expected_value))
> > +               return 0;
> > +       if (bpf_strncmp(value, ret, expected_value))
>
> Hmm. It doesn't work like:
> if (bpf_strncmp(value, ret, "hello"))

This also works. I used expected_value because there is a size
check above. We can also make do something like

if (ret !=3D sizeof("hello"))
             return 0;
if (bpf_strncmp(value, ret, "hello"))
             return 0;

Both of the two work.

Thanks,
Song