Re: [PATCH bpf-next v2 4/6] libbpf: Initialize the bpf_seq_printf parameters array field by field

[Florent Revest <revest@chromium.org>]

On Sat, Mar 27, 2021 at 12:01 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Mar 23, 2021 at 7:23 PM Florent Revest <revest@chromium.org> wrote:
> >
> > When initializing the __param array with a one liner, if all args are
> > const, the initial array value will be placed in the rodata section but
> > because libbpf does not support relocation in the rodata section, any
> > pointer in this array will stay NULL.
> >
> > Fixes: c09add2fbc5a ("tools/libbpf: Add bpf_iter support")
> > Signed-off-by: Florent Revest <revest@chromium.org>
> > ---
> >  tools/lib/bpf/bpf_tracing.h | 26 ++++++++++++++++++++++----
> >  1 file changed, 22 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/lib/bpf/bpf_tracing.h b/tools/lib/bpf/bpf_tracing.h
> > index f9ef37707888..d9a4c3f77ff4 100644
> > --- a/tools/lib/bpf/bpf_tracing.h
> > +++ b/tools/lib/bpf/bpf_tracing.h
> > @@ -413,6 +413,22 @@ typeof(name(0)) name(struct pt_regs *ctx)                              \
> >  }                                                                          \
> >  static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
> >
> > +#define ___bpf_fill0(arr, p, x)
>
> can you please double-check that no-argument BPF_SEQ_PRINTF won't
> generate a warning about spurious ';'? Maybe it's better to have zero
> case as `do {} while(0);` ?
>
> > +#define ___bpf_fill1(arr, p, x) arr[p] = x
> > +#define ___bpf_fill2(arr, p, x, args...) arr[p] = x; ___bpf_fill1(arr, p + 1, args)
> > +#define ___bpf_fill3(arr, p, x, args...) arr[p] = x; ___bpf_fill2(arr, p + 1, args)
> > +#define ___bpf_fill4(arr, p, x, args...) arr[p] = x; ___bpf_fill3(arr, p + 1, args)
> > +#define ___bpf_fill5(arr, p, x, args...) arr[p] = x; ___bpf_fill4(arr, p + 1, args)
> > +#define ___bpf_fill6(arr, p, x, args...) arr[p] = x; ___bpf_fill5(arr, p + 1, args)
> > +#define ___bpf_fill7(arr, p, x, args...) arr[p] = x; ___bpf_fill6(arr, p + 1, args)
> > +#define ___bpf_fill8(arr, p, x, args...) arr[p] = x; ___bpf_fill7(arr, p + 1, args)
> > +#define ___bpf_fill9(arr, p, x, args...) arr[p] = x; ___bpf_fill8(arr, p + 1, args)
> > +#define ___bpf_fill10(arr, p, x, args...) arr[p] = x; ___bpf_fill9(arr, p + 1, args)
> > +#define ___bpf_fill11(arr, p, x, args...) arr[p] = x; ___bpf_fill10(arr, p + 1, args)
> > +#define ___bpf_fill12(arr, p, x, args...) arr[p] = x; ___bpf_fill11(arr, p + 1, args)
> > +#define ___bpf_fill(arr, args...) \
> > +       ___bpf_apply(___bpf_fill, ___bpf_narg(args))(arr, 0, args)
>
> cool. this is regular enough to easily comprehend :)
>
> > +
> >  /*
> >   * BPF_SEQ_PRINTF to wrap bpf_seq_printf to-be-printed values
> >   * in a structure.
> > @@ -421,12 +437,14 @@ static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
> >         ({                                                                  \
> >                 _Pragma("GCC diagnostic push")                              \
> >                 _Pragma("GCC diagnostic ignored \"-Wint-conversion\"")      \
> > +               unsigned long long ___param[___bpf_narg(args)];             \
> >                 static const char ___fmt[] = fmt;                           \
> > -               unsigned long long ___param[] = { args };                   \
> > +               int __ret;                                                  \
> > +               ___bpf_fill(___param, args);                                \
> >                 _Pragma("GCC diagnostic pop")                               \
>
> Let's clean this up a little bit;
> 1. static const char ___fmt should be the very first
> 2. _Pragma scope should be minimal necessary, which includes only
> ___bpf_fill, right?
> 3. Empty line after int __ret; and let's keep three underscores for consistency.
>
>
> > -               int ___ret = bpf_seq_printf(seq, ___fmt, sizeof(___fmt),    \
> > -                                           ___param, sizeof(___param));    \
> > -               ___ret;                                                     \
> > +               __ret = bpf_seq_printf(seq, ___fmt, sizeof(___fmt),         \
> > +                                      ___param, sizeof(___param));         \
> > +               __ret;                                                      \
>
> but actually you don't need __ret at all, just bpf_seq_printf() here, right?

Agreed with everything and also the indentation comment in 5/6, thanks.