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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Apr 6, 2021 at 9:06 AM Florent Revest <revest@chromium.org> wrote:
>
> 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.

So you'd have different bpf_snprintf_proto definitions for sleepable
and non-sleepable programs. And each implementation would call
bpf_printf_prepare() with a flag specifying which copy_string variant
to use (sleepable or not). So for BPF users it would be the same
bpf_snprintf() helper, but it would transparently be doing different
things depending on which BPF program it is being called from. That's
how we do bpf_get_stack(), for example, see
bpf_get_stack_proto_pe/bpf_get_stack_proto_raw_tp/bpf_get_stack_proto_tp.

But consider that for a follow up, no need to address right now.

>
> > > + *             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
>

So I assume you'll adjust the comment? I always find it confusing when
zero case is allowed but it is not specified what's the behavior is.

> > > + *
> > > + *             Or **-EBUSY** if the per-CPU memory copy buffer is busy.
> > >   */

[...]

> > > +       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.
>

snprintf() seems to be always zero-terminating the string if str_size
> 0, and does nothing if str_size == 0, which is exactly what you
want, so you can just drop that zero termination logic.

> 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.


ARG_CONST_SIZE_OR_ZERO should make sure that ARG_PTR_TO_MEM before
that is a valid initialized memory. But please double-check, of
course.