On Thu, Sep 2, 2021 at 10:23 AM Dave Marchevsky <davemarchevsky@xxxxxx> 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@xxxxxx>
> ---
Minor nit below, otherwise looks good:
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> 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
>
