Re: [PATCH bpf-next v2 5/5] selftest/bpf: add test for var-offset stack access

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sun, Jan 24, 2021 at 11:54 AM Andrei Matei <andreimatei1@gmail.com> wrote:
>
> Add a higher-level test (C BPF program) for the new functionality -
> variable access stack reads and writes.
>
> Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
> ---
>  .../selftests/bpf/prog_tests/stack_var_off.c  | 56 +++++++++++++++++++
>  .../selftests/bpf/progs/test_stack_var_off.c  | 43 ++++++++++++++
>  2 files changed, 99 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/stack_var_off.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_stack_var_off.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/stack_var_off.c b/tools/testing/selftests/bpf/prog_tests/stack_var_off.c
> new file mode 100644
> index 000000000000..c4c47fb0f0af
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/stack_var_off.c
> @@ -0,0 +1,56 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <test_progs.h>
> +#include "test_stack_var_off.skel.h"
> +
> +int dummy;
> +
> +noinline void uprobed_function(char *s, int len)
> +{
> +       /* Do something to keep the compiler from removing the function.
> +        */
> +       dummy++;
> +}
> +
> +void test_stack_var_off(void)
> +{
> +       int duration = 0;
> +       struct bpf_link *uprobe_link;
> +       struct test_stack_var_off *skel;
> +       size_t uprobe_offset;
> +       ssize_t base_addr;
> +       char s[100];
> +
> +       base_addr = get_base_addr();
> +       if (CHECK(base_addr < 0, "get_base_addr",
> +                 "failed to find base addr: %zd", base_addr))
> +               return;
> +       uprobe_offset = (size_t)&uprobed_function - base_addr;
> +
> +       skel = test_stack_var_off__open_and_load();
> +       if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
> +               return;
> +       if (CHECK(!skel->bss, "check_bss", ".bss wasn't mmap()-ed\n"))
> +               goto cleanup;
> +
> +       uprobe_link = bpf_program__attach_uprobe(skel->progs.uprobe,
> +                                                false /* retprobe */,
> +                                                0 /* self pid */,
> +                                                "/proc/self/exe",
> +                                                uprobe_offset);
> +       if (CHECK(IS_ERR(uprobe_link), "attach_uprobe",
> +                 "err %ld\n", PTR_ERR(uprobe_link)))
> +               goto cleanup;
> +       skel->links.uprobe = uprobe_link;
> +
> +       /* trigger uprobe */
> +       s[0] = 1;
> +       s[1] = 10;
> +       uprobed_function(&s[0], 2);

I don't think uprobe() is essential to this test and just obscured
what is being tested. I'd just use a global variable to pass whatever
input data you need and use usleep(1), just like lots of other tests.

> +
> +       if (CHECK(skel->bss->uprobe_res != 10, "check_uprobe_res",
> +                 "wrong uprobe res: %d\n", skel->bss->uprobe_res))
> +               goto cleanup;
> +
> +cleanup:
> +       test_stack_var_off__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_stack_var_off.c b/tools/testing/selftests/bpf/progs/test_stack_var_off.c
> new file mode 100644
> index 000000000000..44f982684541
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_stack_var_off.c
> @@ -0,0 +1,43 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright (c) 2017 Facebook
> +
> +#include <linux/ptrace.h>
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +int uprobe_res;
> +
> +SEC("uprobe/func")
> +int BPF_KPROBE(uprobe, char *s, int len)
> +{
> +       /* This BPF program performs variable-offset reads and writes on a
> +        * stack-allocated buffer.
> +        */
> +       char buf[16];
> +       unsigned long idx;
> +       char out;
> +
> +       /* Zero-out the buffer so we can read anywhere inside it. */
> +       __builtin_memset(&buf, 0, 16);
> +       /* Copy the contents of s from user-space. */
> +       len &= 0xf;
> +       if (bpf_probe_read_user(&buf, len, s)) {
> +               bpf_printk("error reading user mem\n");
> +               return 1;
> +       }
> +       /* Index into the buffer at an unknown offset that comes from the
> +        * buffer itself. This is a variable-offset stack read.
> +        */
> +       idx = buf[0];
> +       idx &= 0xf;
> +       out = buf[idx];
> +       /* Append something to the buffer. The position where we append it
> +        * is unknown. This is a variable-offset stack write.
> +        */
> +       buf[len] = buf[idx];
> +       uprobe_res = out;
> +       return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> --
> 2.27.0
>