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

[Andrii Nakryiko]

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


[...]

> +       /* Maximumly we can have MAX_SNPRINTF_VARARGS parameters, just give
> +        * all of them to snprintf().
> +        */
> +       err = snprintf(str, str_size, fmt, BPF_CAST_FMT_ARG(0, args, mod),
> +               BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod),
> +               BPF_CAST_FMT_ARG(3, args, mod), BPF_CAST_FMT_ARG(4, args, mod),
> +               BPF_CAST_FMT_ARG(5, args, mod), BPF_CAST_FMT_ARG(6, args, mod),
> +               BPF_CAST_FMT_ARG(7, args, mod), BPF_CAST_FMT_ARG(8, args, mod),
> +               BPF_CAST_FMT_ARG(9, args, mod), BPF_CAST_FMT_ARG(10, args, mod),
> +               BPF_CAST_FMT_ARG(11, args, mod));
> +
> +       put_fmt_tmp_buf();

reading this for at least 3rd time, this put_fmt_tmp_buf() looks a bit
out of place and kind of random. I think bpf_printf_cleanup() name
pairs with bpf_printf_prepare() better.

> +
> +       return err + 1;

snprintf() already returns string length *including* terminating zero,
so this is wrong


> +}
> +

[...]