Re: [PATCH bpf-next v3 3/6] bpf: Add a bpf_snprintf helper

[Geert Uytterhoeven]

Hi Andrii,

On Wed, Apr 14, 2021 at 9:41 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
> On Mon, Apr 12, 2021 at 8:38 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
> > The implementation takes inspiration from the existing bpf_trace_printk
> > helper but there are a few differences:
> >
> > To allow for a large number of format-specifiers, parameters are
> > provided in an array, like in bpf_seq_printf.
> >
> > Because the output string takes two arguments and the array of
> > parameters also takes two arguments, the format string needs to fit in
> > one argument. Thankfully, ARG_PTR_TO_CONST_STR is guaranteed to point to
> > a zero-terminated read-only map so we don't need a format string length
> > arg.
> >
> > Because the format-string is known at verification time, we also do
> > a first pass of format string validation in the verifier logic. This
> > makes debugging easier.
> >
> > Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> > ---
> >  include/linux/bpf.h            |  6 ++++
> >  include/uapi/linux/bpf.h       | 28 +++++++++++++++++++
> >  kernel/bpf/helpers.c           |  2 ++
> >  kernel/bpf/verifier.c          | 41 ++++++++++++++++++++++++++++
> >  kernel/trace/bpf_trace.c       | 50 ++++++++++++++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 28 +++++++++++++++++++
> >  6 files changed, 155 insertions(+)
> >
>
> [...]
>
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 5f46dd6f3383..d4020e5f91ee 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5918,6 +5918,41 @@ static int check_reference_leak(struct bpf_verifier_env *env)
> >         return state->acquired_refs ? -EINVAL : 0;
> >  }
> >
> > +static int check_bpf_snprintf_call(struct bpf_verifier_env *env,
> > +                                  struct bpf_reg_state *regs)
> > +{
> > +       struct bpf_reg_state *fmt_reg = &regs[BPF_REG_3];
> > +       struct bpf_reg_state *data_len_reg = &regs[BPF_REG_5];
> > +       struct bpf_map *fmt_map = fmt_reg->map_ptr;
> > +       int err, fmt_map_off, num_args;
> > +       u64 fmt_addr;
> > +       char *fmt;
> > +
> > +       /* data must be an array of u64 */
> > +       if (data_len_reg->var_off.value % 8)
> > +               return -EINVAL;
> > +       num_args = data_len_reg->var_off.value / 8;
> > +
> > +       /* fmt being ARG_PTR_TO_CONST_STR guarantees that var_off is const
> > +        * and map_direct_value_addr is set.
> > +        */
> > +       fmt_map_off = fmt_reg->off + fmt_reg->var_off.value;
> > +       err = fmt_map->ops->map_direct_value_addr(fmt_map, &fmt_addr,
> > +                                                 fmt_map_off);
> > +       if (err)
> > +               return err;
> > +       fmt = (char *)fmt_addr + fmt_map_off;
> > +
>
> bot complained about lack of (long) cast before fmt_addr, please address

(uintptr_t), I assume?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds