Re: [PATCH v4 bpf-next 4/9] libbpf: Modify bpf_printk to choose helper based on arg count

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

On Thu, Sep 2, 2021 at 10:23 AM Dave Marchevsky <davemarchevsky@fb.com> wrote:
>
> Instead of being a thin wrapper which calls into bpf_trace_printk,
> libbpf's bpf_printk convenience macro now chooses between
> bpf_trace_printk and bpf_trace_vprintk. If the arg count (excluding
> format string) is >3, use bpf_trace_vprintk, otherwise use the older
> helper.
>
> The motivation behind this added complexity - instead of migrating
> entirely to bpf_trace_vprintk - is to maintain good developer experience
> for users compiling against new libbpf but running on older kernels.
> Users who are passing <=3 args to bpf_printk will see no change in their
> bytecode.
>
> __bpf_vprintk functions similarly to BPF_SEQ_PRINTF and BPF_SNPRINTF
> macros elsewhere in the file - it allows use of bpf_trace_vprintk
> without manual conversion of varargs to u64 array. Previous
> implementation of bpf_printk macro is moved to __bpf_printk for use by
> the new implementation.
>
> This does change behavior of bpf_printk calls with >3 args in the "new
> libbpf, old kernels" scenario. Before this patch, attempting to use 4
> args to bpf_printk results in a compile-time error. After this patch,
> using bpf_printk with 4 args results in a trace_vprintk helper call
> being emitted and a load-time failure on older kernels.
>
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> ---

Minor nit below, otherwise looks good:

Acked-by: Andrii Nakryiko <andrii@kernel.org>

>  tools/lib/bpf/bpf_helpers.h | 45 ++++++++++++++++++++++++++++++-------
>  1 file changed, 37 insertions(+), 8 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index b9987c3efa3c..a7e73be6dac4 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -14,14 +14,6 @@
>  #define __type(name, val) typeof(val) *name
>  #define __array(name, val) typeof(val) *name[]
>
> -/* Helper macro to print out debug messages */
> -#define bpf_printk(fmt, ...)                           \
> -({                                                     \
> -       char ____fmt[] = fmt;                           \
> -       bpf_trace_printk(____fmt, sizeof(____fmt),      \
> -                        ##__VA_ARGS__);                \
> -})
> -
>  /*
>   * Helper macro to place programs, maps, license in
>   * different sections in elf_bpf file. Section names
> @@ -224,4 +216,41 @@ enum libbpf_tristate {
>                      ___param, sizeof(___param));               \
>  })
>
> +/* Helper macro to print out debug messages */

this comment probably should have been moved into the new bpf_printk() macro...

> +#define __bpf_printk(fmt, ...)                         \
> +({                                                     \
> +       char ____fmt[] = fmt;                           \
> +       bpf_trace_printk(____fmt, sizeof(____fmt),      \
> +                        ##__VA_ARGS__);                \
> +})
> +
> +/*
> + * __bpf_vprintk wraps the bpf_trace_vprintk helper with variadic arguments
> + * instead of an array of u64.
> + */
> +#define __bpf_vprintk(fmt, args...)                            \
> +({                                                             \
> +       static const char ___fmt[] = fmt;                       \
> +       unsigned long long ___param[___bpf_narg(args)];         \
> +                                                               \
> +       _Pragma("GCC diagnostic push")                          \
> +       _Pragma("GCC diagnostic ignored \"-Wint-conversion\"")  \
> +       ___bpf_fill(___param, args);                            \
> +       _Pragma("GCC diagnostic pop")                           \
> +                                                               \
> +       bpf_trace_vprintk(___fmt, sizeof(___fmt),               \
> +                         ___param, sizeof(___param));          \
> +})
> +
> +/* Use __bpf_printk when bpf_printk call has 3 or fewer fmt args
> + * Otherwise use __bpf_vprintk
> + */
> +#define ___bpf_pick_printk(...) \
> +       ___bpf_nth(_, ##__VA_ARGS__, __bpf_vprintk, __bpf_vprintk, __bpf_vprintk,       \
> +                  __bpf_vprintk, __bpf_vprintk, __bpf_vprintk, __bpf_vprintk,          \
> +                  __bpf_vprintk, __bpf_vprintk, __bpf_printk /*3*/, __bpf_printk /*2*/,\
> +                  __bpf_printk /*1*/, __bpf_printk /*0*/)
> +
> +#define bpf_printk(fmt, args...) ___bpf_pick_printk(args)(fmt, ##args)
> +
>  #endif
> --
> 2.30.2
>