linux-kernel - Re: [PATCH bpf-next v3 2/3] selftests: bpf: add a test for mmapable vmlinux BTF

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Message-ID: <CAEf4BzboH-au2bNCWYk1nYbQ61kGbUXuvTxftDPAEGF1Pc=TLw@mail.gmail.com>
Date: Tue, 6 May 2025 14:39:18 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Lorenz Bauer <lmb@...valent.com>
Cc: Arnd Bergmann <arnd@...db.de>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 2/3] selftests: bpf: add a test for mmapable
 vmlinux BTF

On Mon, May 5, 2025 at 11:39 AM Lorenz Bauer <lmb@...valent.com> wrote:
>
> Add a basic test for the ability to mmap /sys/kernel/btf/vmlinux. Since
> libbpf doesn't have an API to parse BTF from memory we do some basic
> sanity checks ourselves.
>
> Signed-off-by: Lorenz Bauer <lmb@...valent.com>
> ---
>  tools/testing/selftests/bpf/prog_tests/btf_sysfs.c | 83 ++++++++++++++++++++++
>  1 file changed, 83 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/btf_sysfs.c b/tools/testing/selftests/bpf/prog_tests/btf_sysfs.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..3319cf758897d46cefa8ca25e16acb162f4e9889
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/btf_sysfs.c
> @@ -0,0 +1,83 @@
> +// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
> +/* Copyright (c) 2025 Isovalent */
> +
> +#include <test_progs.h>
> +#include <bpf/btf.h>
> +#include <sys/stat.h>
> +#include <sys/mman.h>
> +#include <fcntl.h>
> +#include <unistd.h>
> +
> +static void test_btf_mmap_sysfs(const char *path, struct btf *base)
> +{
> +       struct stat st;
> +       __u64 btf_size, end;
> +       void *raw_data = NULL;
> +       int fd = -1;
> +       long page_size;
> +       struct btf *btf = NULL;
> +
> +       page_size = sysconf(_SC_PAGESIZE);
> +       if (!ASSERT_GE(page_size, 0, "get_page_size"))
> +               goto cleanup;
> +
> +       if (!ASSERT_OK(stat(path, &st), "stat_btf"))
> +               goto cleanup;
> +
> +       btf_size = st.st_size;
> +       end = (btf_size + page_size - 1) / page_size * page_size;
> +
> +       fd = open(path, O_RDONLY);
> +       if (!ASSERT_GE(fd, 0, "open_btf"))
> +               goto cleanup;
> +
> +       raw_data = mmap(NULL, btf_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
> +       if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_writable"))
> +               goto cleanup;
> +
> +       raw_data = mmap(NULL, btf_size, PROT_READ, MAP_SHARED, fd, 0);
> +       if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_shared"))
> +               goto cleanup;
> +
> +       raw_data = mmap(NULL, end + 1, PROT_READ, MAP_PRIVATE, fd, 0);
> +       if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_invalid_size"))
> +               goto cleanup;
> +
> +       raw_data = mmap(NULL, end, PROT_READ, MAP_PRIVATE, fd, 0);
> +       if (!ASSERT_NEQ(raw_data, MAP_FAILED, "mmap_btf"))

ASSERT_OK_PTR()?

> +               goto cleanup;
> +
> +       if (!ASSERT_EQ(mprotect(raw_data, btf_size, PROT_READ | PROT_WRITE), -1,
> +           "mprotect_writable"))
> +               goto cleanup;
> +
> +       if (!ASSERT_EQ(mprotect(raw_data, btf_size, PROT_READ | PROT_EXEC), -1,
> +           "mprotect_executable"))
> +               goto cleanup;
> +
> +       /* Check padding is zeroed */
> +       for (int i = btf_size; i < end; i++) {
> +               if (((__u8 *)raw_data)[i] != 0) {
> +                       PRINT_FAIL("tail of BTF is not zero at page offset %d\n", i);
> +                       goto cleanup;
> +               }
> +       }
> +
> +       btf = btf__new_split(raw_data, btf_size, base);
> +       if (!ASSERT_NEQ(btf, NULL, "parse_btf"))

ASSERT_OK_PTR()

> +               goto cleanup;
> +
> +cleanup:
> +       if (raw_data && raw_data != MAP_FAILED)
> +               munmap(raw_data, btf_size);
> +       if (btf)

no need to check this, all libbpf destructor APIs deal with NULL
correctly (ignoring them)

> +               btf__free(btf);
> +       if (fd >= 0)
> +               close(fd);
> +}
> +
> +void test_btf_sysfs(void)
> +{
> +       if (test__start_subtest("vmlinux"))
> +               test_btf_mmap_sysfs("/sys/kernel/btf/vmlinux", NULL);

Do you intend to add more subtests? if not, why even using a subtest structure

> +}

>
> --
> 2.49.0
>