Re: [PATCH v5 bpf-next 4/4] selftest/bpf: Extend the bpf_snprintf() test for "%c".

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

On Thu, Aug 12, 2021 at 9:47 AM Kuniyuki Iwashima <kuniyu@amazon.co.jp> wrote:
>
> This patch adds a "positive" pattern for "%c", which intentionally uses a
> __u32 value (0x64636261, "dbca") to print a single character "a".  If the
> implementation went wrong, other 3 bytes might show up as the part of the
> latter "%+05s".
>
> Also, this patch adds two "negative" patterns for wide character.
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
> ---
>  tools/testing/selftests/bpf/prog_tests/snprintf.c | 4 +++-
>  tools/testing/selftests/bpf/progs/test_snprintf.c | 7 ++++---
>  2 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/snprintf.c b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> index dffbcaa1ec98..f77d7def7fed 100644
> --- a/tools/testing/selftests/bpf/prog_tests/snprintf.c
> +++ b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> @@ -19,7 +19,7 @@
>  #define EXP_ADDR_OUT "0000000000000000 ffff00000add4e55 "
>  #define EXP_ADDR_RET sizeof(EXP_ADDR_OUT "unknownhashedptr")
>
> -#define EXP_STR_OUT  "str1 longstr"
> +#define EXP_STR_OUT  "str1         a longstr"
>  #define EXP_STR_RET  sizeof(EXP_STR_OUT)
>
>  #define EXP_OVER_OUT "%over"
> @@ -114,6 +114,8 @@ void test_snprintf_negative(void)
>         ASSERT_ERR(load_single_snprintf("%"), "invalid specifier 3");
>         ASSERT_ERR(load_single_snprintf("%12345678"), "invalid specifier 4");
>         ASSERT_ERR(load_single_snprintf("%--------"), "invalid specifier 5");
> +       ASSERT_ERR(load_single_snprintf("%lc"), "invalid specifier 6");
> +       ASSERT_ERR(load_single_snprintf("%llc"), "invalid specifier 7");
>         ASSERT_ERR(load_single_snprintf("\x80"), "non ascii character");
>         ASSERT_ERR(load_single_snprintf("\x1"), "non printable character");
>  }
> diff --git a/tools/testing/selftests/bpf/progs/test_snprintf.c b/tools/testing/selftests/bpf/progs/test_snprintf.c
> index e2ad26150f9b..afc2c583125b 100644
> --- a/tools/testing/selftests/bpf/progs/test_snprintf.c
> +++ b/tools/testing/selftests/bpf/progs/test_snprintf.c
> @@ -40,6 +40,7 @@ int handler(const void *ctx)
>         /* Convenient values to pretty-print */
>         const __u8 ex_ipv4[] = {127, 0, 0, 1};
>         const __u8 ex_ipv6[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1};
> +       const __u32 chr1 = 0x64636261; /* dcba */
>         static const char str1[] = "str1";
>         static const char longstr[] = "longstr";
>
> @@ -59,9 +60,9 @@ int handler(const void *ctx)
>         /* Kernel pointers */
>         addr_ret = BPF_SNPRINTF(addr_out, sizeof(addr_out), "%pK %px %p",
>                                 0, 0xFFFF00000ADD4E55, 0xFFFF00000ADD4E55);
> -       /* Strings embedding */
> -       str_ret  = BPF_SNPRINTF(str_out, sizeof(str_out), "%s %+05s",
> -                               str1, longstr);
> +       /* Strings and single-byte character embedding */
> +       str_ret  = BPF_SNPRINTF(str_out, sizeof(str_out), "%s % 9c %+05s",
> +                               str1, chr1, longstr);


Why this hackery with __u32? You are making an endianness assumption
(it will break on big-endian), and you'd never write real code like
that. Just pass 'a', what's wrong with that?

>         /* Overflow */
>         over_ret = BPF_SNPRINTF(over_out, sizeof(over_out), "%%overflow");
>         /* Padding of fixed width numbers */
> --
> 2.30.2
>