Re: [PATCH bpf-next v2 3/6] bpf: Add a bpf_snprintf helper

[Florent Revest <revest@chromium.org>]

On Fri, Mar 26, 2021 at 11:55 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
> On Tue, Mar 23, 2021 at 7:23 PM Florent Revest <revest@chromium.org> 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. But because ARG_PTR_TO_CONST_STR guarantees to point to a
> > NULL-terminated read-only map, we don't need a format string length arg.
> >
> > Because the format-string is known at verification time, we also move
> > most of the format string validation, currently done in formatting
> > helper calls, into the verifier logic. This makes debugging easier and
> > also slightly improves the runtime performance.
> >
> > Signed-off-by: Florent Revest <revest@chromium.org>
> > ---
> >  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       | 52 ++++++++++++++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 28 ++++++++++++++++++
> >  6 files changed, 157 insertions(+)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 7b5319d75b3e..f3d9c8fa60b3 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1893,6 +1893,7 @@ extern const struct bpf_func_proto bpf_skc_to_tcp_request_sock_proto;
> >  extern const struct bpf_func_proto bpf_skc_to_udp6_sock_proto;
> >  extern const struct bpf_func_proto bpf_copy_from_user_proto;
> >  extern const struct bpf_func_proto bpf_snprintf_btf_proto;
> > +extern const struct bpf_func_proto bpf_snprintf_proto;
> >  extern const struct bpf_func_proto bpf_per_cpu_ptr_proto;
> >  extern const struct bpf_func_proto bpf_this_cpu_ptr_proto;
> >  extern const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto;
> > @@ -2018,4 +2019,9 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
> >  struct btf_id_set;
> >  bool btf_id_set_contains(const struct btf_id_set *set, u32 id);
> >
> > +enum bpf_printf_mod_type;
> > +int bpf_printf_preamble(char *fmt, u32 fmt_size, const u64 *raw_args,
> > +                       u64 *final_args, enum bpf_printf_mod_type *mod,
> > +                       u32 num_args);
> > +
> >  #endif /* _LINUX_BPF_H */
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 2d3036e292a9..86af61e912c6 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -4660,6 +4660,33 @@ union bpf_attr {
> >   *     Return
> >   *             The number of traversed map elements for success, **-EINVAL** for
> >   *             invalid **flags**.
> > + *
> > + * long bpf_snprintf(char *str, u32 str_size, const char *fmt, u64 *data, u32 data_len)
> > + *     Description
> > + *             Outputs a string into the **str** buffer of size **str_size**
> > + *             based on a format string stored in a read-only map pointed by
> > + *             **fmt**.
> > + *
> > + *             Each format specifier in **fmt** corresponds to one u64 element
> > + *             in the **data** array. For strings and pointers where pointees
> > + *             are accessed, only the pointer values are stored in the *data*
> > + *             array. The *data_len* is the size of *data* in bytes.
> > + *
> > + *             Formats **%s** and **%p{i,I}{4,6}** require to read kernel
> > + *             memory. Reading kernel memory may fail due to either invalid
> > + *             address or valid address but requiring a major memory fault. If
> > + *             reading kernel memory fails, the string for **%s** will be an
> > + *             empty string, and the ip address for **%p{i,I}{4,6}** will be 0.
>
> would it make sense for sleepable programs to allow memory fault when
> reading memory?

Probably yes. How would you do that ? I'm guessing that in
bpf_trace_copy_string you would call either strncpy_from_X_nofault or
strncpy_from_X depending on a condition but I'm not sure which one.

> > + *             Not returning error to bpf program is consistent with what
> > + *             **bpf_trace_printk**\ () does for now.
> > + *
> > + *     Return
> > + *             The strictly positive length of the formatted string, including
> > + *             the trailing zero character. If the return value is greater than
> > + *             **str_size**, **str** contains a truncated string, guaranteed to
> > + *             be zero-terminated.
>
> Except when str_size == 0.

Right

> > + *
> > + *             Or **-EBUSY** if the per-CPU memory copy buffer is busy.
> >   */
> >  #define __BPF_FUNC_MAPPER(FN)          \
> >         FN(unspec),                     \
> > @@ -4827,6 +4854,7 @@ union bpf_attr {
> >         FN(sock_from_file),             \
> >         FN(check_mtu),                  \
> >         FN(for_each_map_elem),          \
> > +       FN(snprintf),                   \
> >         /* */
> >
> >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 074800226327..12f4cfb04fe7 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -750,6 +750,8 @@ bpf_base_func_proto(enum bpf_func_id func_id)
> >                 return &bpf_probe_read_kernel_str_proto;
> >         case BPF_FUNC_snprintf_btf:
> >                 return &bpf_snprintf_btf_proto;
> > +       case BPF_FUNC_snprintf:
> > +               return &bpf_snprintf_proto;
> >         default:
> >                 return NULL;
> >         }
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 9e03608725b4..a89599dc51c9 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5729,6 +5729,41 @@ 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 so data_len must be a multiple of 8 */
> > +       if (data_len_reg->var_off.value & 7)
>
> `% 8` is not cool anymore... :)

Haha, this is a leftover from bpf_seq_printf but I agree % 8 is nicer.

> > +               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;
> > +
> > +       /* We are also guaranteed that fmt+fmt_map_off is NULL terminated, we
> > +        * can focus on validating the format specifiers.
> > +        */
> > +       err = bpf_printf_preamble(fmt, UINT_MAX, NULL, NULL, NULL, num_args);
> > +       if (err < 0)
> > +               verbose(env, "Invalid format string\n");
> > +
> > +       return err;
> > +}
> > +
> >  static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> >                              int *insn_idx_p)
> >  {
> > @@ -5843,6 +5878,12 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> >                         return -EINVAL;
> >         }
> >
> > +       if (func_id == BPF_FUNC_snprintf) {
> > +               err = check_bpf_snprintf_call(env, regs);
> > +               if (err < 0)
> > +                       return err;
> > +       }
> > +
> >         /* reset caller saved regs */
> >         for (i = 0; i < CALLER_SAVED_REGS; i++) {
> >                 mark_reg_not_init(env, regs, caller_saved[i]);
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 0fdca94a3c9c..15cbc8b63206 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1230,6 +1230,56 @@ const struct bpf_func_proto bpf_snprintf_btf_proto = {
> >         .arg5_type      = ARG_ANYTHING,
> >  };
> >
> > +#define MAX_SNPRINTF_VARARGS           12
> > +
> > +BPF_CALL_5(bpf_snprintf, char *, str, u32, str_size, char *, fmt,
> > +          const void *, data, u32, data_len)
> > +{
> > +       enum bpf_printf_mod_type mod[MAX_SNPRINTF_VARARGS];
> > +       u64 args[MAX_SNPRINTF_VARARGS];
> > +       int err, num_args;
> > +
> > +       if (data_len & 7 || data_len > MAX_SNPRINTF_VARARGS * 8 ||
> > +           (data_len && !data))
>
> see previous patches, data_len > 0 should be iff data != NULL, I think

Commented there.

> > +               return -EINVAL;
> > +       num_args = data_len / 8;
> > +
> > +       /* ARG_PTR_TO_CONST_STR guarantees that fmt is zero-terminated so we
> > +        * can safely give an unbounded size.
> > +        */
> > +       err = bpf_printf_preamble(fmt, UINT_MAX, data, args, mod, num_args);
> > +       if (err < 0)
> > +               return err;
> > +
> > +       /* 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));
> > +       if (str_size)
> > +               str[str_size - 1] = '\0';
>
> hm... what if err < str_size ?

Then there would be two zeroes, one set by snprintf in the middle and
one set by us at the end. :| I was a bit lazy there, I agree it would
be nicer if we'd do if (err >= str_size) instead.

Also makes me wonder what if str == NULL and str_size != 0. I just
assumed that the verifier would prevent that from happening but
discussions in the other patches make me unsure now.