Re: [PATCH v5 3/3] libbpf: Add a test to confirm PT_REGS_PARM4_SYSCALL

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Kenta Tada <Kenta.Tada@sony.com>, Hengqi Chen <hengqi.chen@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>, Martin Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	john fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>
Subject: Re: [PATCH v5 3/3] libbpf: Add a test to confirm PT_REGS_PARM4_SYSCALL
Date: Mon, 24 Jan 2022 21:05:38 -0800	[thread overview]
Message-ID: <CAEf4BzZGARxqmCFmGhJduAu+Wsg2t0RVHLXrfX=KuHuhnhv+OA@mail.gmail.com> (raw)
In-Reply-To: <20220124141622.4378-4-Kenta.Tada@sony.com>

+cc Hengqi

On Mon, Jan 24, 2022 at 6:20 AM Kenta Tada <Kenta.Tada@sony.com> wrote:
>
> Add a selftest to verify the behavior of PT_REGS_xxx
> and the CORE variant.
>
> Signed-off-by: Kenta Tada <Kenta.Tada@sony.com>
> ---
>  .../bpf/prog_tests/test_bpf_syscall_macro.c   | 63 ++++++++++++++++++
>  .../selftests/bpf/progs/bpf_syscall_macro.c   | 64 +++++++++++++++++++
>  2 files changed, 127 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/test_bpf_syscall_macro.c
>  create mode 100644 tools/testing/selftests/bpf/progs/bpf_syscall_macro.c
>

[...]

> diff --git a/tools/testing/selftests/bpf/progs/bpf_syscall_macro.c b/tools/testing/selftests/bpf/progs/bpf_syscall_macro.c
> new file mode 100644
> index 000000000000..cfeccd85f40e
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/bpf_syscall_macro.c
> @@ -0,0 +1,64 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright 2022 Sony Group Corporation */
> +#include <vmlinux.h>
> +
> +#include <bpf/bpf_core_read.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +#include "bpf_misc.h"
> +
> +int arg1 = 0;
> +unsigned long arg2 = 0;
> +unsigned long arg3 = 0;
> +unsigned long arg4_cx = 0;
> +unsigned long arg4 = 0;
> +unsigned long arg5 = 0;
> +
> +int arg1_core = 0;
> +unsigned long arg2_core = 0;
> +unsigned long arg3_core = 0;
> +unsigned long arg4_core_cx = 0;
> +unsigned long arg4_core = 0;
> +unsigned long arg5_core = 0;
> +
> +const volatile pid_t filter_pid = 0;
> +
> +SEC("kprobe/" SYS_PREFIX "sys_prctl")
> +int BPF_KPROBE(handle_sys_prctl)
> +{
> +       struct pt_regs *real_regs;
> +       int orig_arg1;
> +       unsigned long orig_arg2, orig_arg3, orig_arg4_cx, orig_arg4, orig_arg5;
> +       pid_t pid = bpf_get_current_pid_tgid() >> 32;
> +
> +       if (pid != filter_pid)
> +               return 0;
> +
> +       /* test for PT_REGS_PARM */
> +       real_regs = (struct pt_regs *)PT_REGS_PARM1(ctx);
> +       bpf_probe_read_kernel(&orig_arg1, sizeof(orig_arg1), &PT_REGS_PARM1_SYSCALL(real_regs));
> +       bpf_probe_read_kernel(&orig_arg2, sizeof(orig_arg2), &PT_REGS_PARM2_SYSCALL(real_regs));
> +       bpf_probe_read_kernel(&orig_arg3, sizeof(orig_arg3), &PT_REGS_PARM3_SYSCALL(real_regs));
> +       bpf_probe_read_kernel(&orig_arg4_cx, sizeof(orig_arg4_cx), &PT_REGS_PARM4(real_regs));
> +       bpf_probe_read_kernel(&orig_arg4, sizeof(orig_arg4), &PT_REGS_PARM4_SYSCALL(real_regs));
> +       bpf_probe_read_kernel(&orig_arg5, sizeof(orig_arg5), &PT_REGS_PARM5_SYSCALL(real_regs));
> +       /* copy all actual args and the wrong arg4 on x86_64 */
> +       arg1 = orig_arg1;
> +       arg2 = orig_arg2;
> +       arg3 = orig_arg3;
> +       arg4_cx = orig_arg4_cx;
> +       arg4 = orig_arg4;
> +       arg5 = orig_arg5;

I don't get why you needed orig_argX variables and then copying them
into argX variables. I changed this to read directly into argX. I
suspect arg1 handling might break on big-endian arches due to int vs
long differences, please check that and send a follow up fix.

Also keep in mind that selftest changes should come with
"selftests/bpf:" subject prefix, not "libbpf:". Fixed that up as well.

Applied to bpf-next, thanks.

Hopefully Hengqi will build his SYSCALL prog wrapper macros on top of
this as a follow up as well.

> +
> +       /* test for the CORE variant of PT_REGS_PARM */
> +       arg1_core = PT_REGS_PARM1_CORE_SYSCALL(real_regs);
> +       arg2_core = PT_REGS_PARM2_CORE_SYSCALL(real_regs);
> +       arg3_core = PT_REGS_PARM3_CORE_SYSCALL(real_regs);
> +       arg4_core_cx = PT_REGS_PARM4_CORE(real_regs);
> +       arg4_core = PT_REGS_PARM4_CORE_SYSCALL(real_regs);
> +       arg5_core = PT_REGS_PARM5_CORE_SYSCALL(real_regs);
> +
> +       return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> --
> 2.32.0
>