Re: [PATCH v6 bpf-next 1/3] libbpf: BTF dumper support for typed data

[Andrii Nakryiko]

On Thu, Jul 15, 2021 at 11:24 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Jul 15, 2021 at 8:15 AM Alan Maguire <alan.maguire@xxxxxxxxxx> wrote:
> >
> > Add a BTF dumper for typed data, so that the user can dump a typed
> > version of the data provided.
> >
> > The API is
> >
> > int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
> >                              void *data, size_t data_sz,
> >                              const struct btf_dump_type_data_opts *opts);
> >
> > ...where the id is the BTF id of the data pointed to by the "void *"
> > argument; for example the BTF id of "struct sk_buff" for a
> > "struct skb *" data pointer.  Options supported are
> >
> >  - a starting indent level (indent_lvl)
> >  - a user-specified indent string which will be printed once per
> >    indent level; if NULL, tab is chosen but any string <= 32 chars
> >    can be provided.
> >  - a set of boolean options to control dump display, similar to those
> >    used for BPF helper bpf_snprintf_btf().  Options are
> >         - compact : omit newlines and other indentation
> >         - skip_names: omit member names
> >         - emit_zeroes: show zero-value members
> >
> > Default output format is identical to that dumped by bpf_snprintf_btf(),
> > for example a "struct sk_buff" representation would look like this:
> >
> > struct sk_buff){
> >         (union){
> >                 (struct){
> >                         .next = (struct sk_buff *)0xffffffffffffffff,
> >                         .prev = (struct sk_buff *)0xffffffffffffffff,
> >                 (union){
> >                         .dev = (struct net_device *)0xffffffffffffffff,
> >                         .dev_scratch = (long unsigned int)18446744073709551615,
> >                 },
> >         },
> > ...
> >
> > If the data structure is larger than the *data_sz*
> > number of bytes that are available in *data*, as much
> > of the data as possible will be dumped and -E2BIG will
> > be returned.  This is useful as tracers will sometimes
> > not be able to capture all of the data associated with
> > a type; for example a "struct task_struct" is ~16k.
> > Being able to specify that only a subset is available is
> > important for such cases.  On success, the amount of data
> > dumped is returned.
> >
> > Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> > ---
>
> Ok, this looks great. I think I found a few residual problems, so
> please see comments below and address them. But I'm inclined to land
> this patch set as is because it's in a good shape already, and it is
> pretty, so it's hard and time-consuming to weed through minor (at this
> point) changes between versions. So please send follow-up patch(es)
> with fixes. Hopefully soon enough before the libbpf release. Thanks a
> lot for working on this and persevering, this is a great API!
>
> I'll apply a patch set to bpf-next when it will open up for new patches. Thanks.

Applied to bpf-next.

>
> >  tools/lib/bpf/btf.h      |  19 ++
> >  tools/lib/bpf/btf_dump.c | 819 ++++++++++++++++++++++++++++++++++++++++++++++-
> >  tools/lib/bpf/libbpf.map |   1 +
> >  3 files changed, 834 insertions(+), 5 deletions(-)
>
> I also wanted to call out this ^^ versus:
>
> a) initial kernel-sharing version:
>
>   >  18 files changed, 3236 insertions(+), 1319 deletions(-)
>
> b) initial libbpf-only version:
>
>   >  6 files changed, 1251 insertions(+), 3 deletions(-)
>
> And the API actually gained in supported features and correctness.
>
> >
>
> [...]
>
> > +
> > +union float_data {
> > +       long double ld;
> > +       double d;
> > +       float f;
> > +};
>
> clever
>
> > +
> > +static int btf_dump_float_data(struct btf_dump *d,
> > +                              const struct btf_type *t,
> > +                              __u32 type_id,
> > +                              const void *data)
> > +{
> > +       const union float_data *flp = data;
> > +       union float_data fl;
> > +       int sz = t->size;
> > +
> > +       /* handle unaligned data; copy to local union */
> > +       if (((uintptr_t)data) % sz) {
> > +               memcpy(&fl, data, sz);
> > +               flp = &fl;
> > +       }
> > +
> > +       switch (sz) {
> > +       case 16:
> > +               btf_dump_type_values(d, "%Lf", flp->ld);
> > +               break;
> > +       case 8:
> > +               btf_dump_type_values(d, "%lf", flp->d);
> > +               break;
> > +       case 4:
> > +               btf_dump_type_values(d, "%f", flp->f);
> > +               break;
> > +       default:
> > +               pr_warn("unexpected size %d for id [%u]\n", sz, type_id);
> > +               return -EINVAL;
> > +       }
> > +       return 0;
> > +}
> > +
>
> [...]
>
> > +
> > +static int btf_dump_array_data(struct btf_dump *d,
> > +                              const struct btf_type *t,
> > +                              __u32 id,
> > +                              const void *data)
> > +{
> > +       const struct btf_array *array = btf_array(t);
> > +       const struct btf_type *elem_type;
> > +       __u32 i, elem_size = 0, elem_type_id;
> > +       bool is_array_member;
> > +
> > +       elem_type_id = array->type;
> > +       elem_type = skip_mods_and_typedefs(d->btf, elem_type_id, NULL);
> > +       elem_size = btf__resolve_size(d->btf, elem_type_id);
> > +       if (elem_size <= 0) {
> > +               pr_warn("unexpected elem size %d for array type [%u]\n", elem_size, id);
> > +               return -EINVAL;
> > +       }
> > +
> > +       if (btf_is_int(elem_type)) {
> > +               /*
> > +                * BTF_INT_CHAR encoding never seems to be set for
> > +                * char arrays, so if size is 1 and element is
> > +                * printable as a char, we'll do that.
> > +                */
> > +               if (elem_size == 1)
> > +                       d->typed_dump->is_array_char = true;
> > +       }
> > +
> > +       /* note that we increment depth before calling btf_dump_print() below;
> > +        * this is intentional.  btf_dump_data_newline() will not print a
> > +        * newline for depth 0 (since this leaves us with trailing newlines
> > +        * at the end of typed display), so depth is incremented first.
> > +        * For similar reasons, we decrement depth before showing the closing
> > +        * parenthesis.
> > +        */
> > +       d->typed_dump->depth++;
> > +       btf_dump_printf(d, "[%s", btf_dump_data_newline(d));
> > +
> > +       /* may be a multidimensional array, so store current "is array member"
> > +        * status so we can restore it correctly later.
> > +        */
> > +       is_array_member = d->typed_dump->is_array_member;
> > +       d->typed_dump->is_array_member = true;
> > +       for (i = 0; i < array->nelems; i++, data += elem_size) {
> > +               if (d->typed_dump->is_array_terminated)
> > +                       break;
>
> I suspect this logic breaks for multi-dimensional char arrays. Please
> check and add follow-up tests and fixes, no need to address that in
> this patch set, you've suffered enough.
>
>
> > +               btf_dump_dump_type_data(d, NULL, elem_type, elem_type_id, data, 0, 0);
> > +       }
> > +       d->typed_dump->is_array_member = is_array_member;
> > +       d->typed_dump->depth--;
> > +       btf_dump_data_pfx(d);
> > +       btf_dump_type_values(d, "]");
> > +
> > +       return 0;
> > +}
> > +
> > +static int btf_dump_struct_data(struct btf_dump *d,
> > +                               const struct btf_type *t,
> > +                               __u32 id,
> > +                               const void *data)
> > +{
> > +       const struct btf_member *m = btf_members(t);
> > +       __u16 n = btf_vlen(t);
> > +       int i, err;
> > +
> > +       /* note that we increment depth before calling btf_dump_print() below;
> > +        * this is intentional.  btf_dump_data_newline() will not print a
> > +        * newline for depth 0 (since this leaves us with trailing newlines
> > +        * at the end of typed display), so depth is incremented first.
> > +        * For similar reasons, we decrement depth before showing the closing
> > +        * parenthesis.
> > +        */
>
> ah, ok, I see. I sort of randomly stumbled on this from a purely
> aesthetic reasons, but I'm happy we clarified this because it's
> completely non-obvious
>
> > +       d->typed_dump->depth++;
> > +       btf_dump_printf(d, "{%s", btf_dump_data_newline(d));
> > +
> > +       for (i = 0; i < n; i++, m++) {
> > +               const struct btf_type *mtype;
> > +               const char *mname;
> > +               __u32 moffset;
> > +               __u8 bit_sz;
> > +
> > +               mtype = btf__type_by_id(d->btf, m->type);
> > +               mname = btf_name_of(d, m->name_off);
> > +               moffset = btf_member_bit_offset(t, i);
> > +
> > +               bit_sz = btf_member_bitfield_size(t, i);
> > +               err = btf_dump_dump_type_data(d, mname, mtype, m->type, data + moffset / 8,
> > +                                             moffset % 8, bit_sz);
> > +               if (err < 0)
> > +                       return err;
> > +       }
> > +       d->typed_dump->depth--;
> > +       btf_dump_data_pfx(d);
> > +       btf_dump_type_values(d, "}");
> > +       return err;
> > +}
> > +
> > +static int btf_dump_ptr_data(struct btf_dump *d,
> > +                             const struct btf_type *t,
> > +                             __u32 id,
> > +                             const void *data)
> > +{
> > +       btf_dump_type_values(d, "%p", *(void **)data);
>
> Wait, you fixed pointer zero checking logic and misaligned reads for
> ints/floats, but none of that for actually printing pointers?...
> Please send a follow-up fix.
>
> > +       return 0;
> > +}
> > +
> > +static int btf_dump_get_enum_value(struct btf_dump *d,
> > +                                  const struct btf_type *t,
> > +                                  const void *data,
> > +                                  __u32 id,
> > +                                  __s64 *value)
> > +{
> > +       int sz = t->size;
> > +
> > +       /* handle unaligned enum value */
> > +       if (((uintptr_t)data) % sz) {
>
> nit: probably worth a small helper with obvious name to avoid extra
> comments and all those ((()))
>
> > +               *value = (__s64)btf_dump_bitfield_get_data(d, t, data, 0, 0);
> > +               return 0;
> > +       }
>
> [...]
>
> > +               elem_type_id = array->type;
> > +               elem_size = btf__resolve_size(d->btf, elem_type_id);
> > +               elem_type = skip_mods_and_typedefs(d->btf, elem_type_id, NULL);
> > +
> > +               ischar = btf_is_int(elem_type) && elem_size == 1;
> > +
> > +               /* check all elements; if _any_ element is nonzero, all
> > +                * of array is displayed.  We make an exception however
> > +                * for char arrays where the first element is 0; these
> > +                * are considered zeroed also, even if later elements are
> > +                * non-zero because the string is terminated.
> > +                */
> > +               for (i = 0; i < array->nelems; i++) {
> > +                       if (i == 0 && ischar && *(char *)data == 0)
> > +                               return -ENODATA;
>
> same here, this might be too aggressive for something like char a[2][10] ?
>
> > +                       err = btf_dump_type_data_check_zero(d, elem_type,
> > +                                                           elem_type_id,
> > +                                                           data +
> > +                                                           (i * elem_size),
> > +                                                           bits_offset, 0);
> > +                       if (err != -ENODATA)
> > +                               return err;
> > +               }
> > +               return -ENODATA;
> > +       }
> > +       case BTF_KIND_STRUCT:
> > +       case BTF_KIND_UNION: {
> > +               const struct btf_member *m = btf_members(t);
> > +               __u16 n = btf_vlen(t);
> > +
> > +               /* if any struct/union member is non-zero, the struct/union
> > +                * is considered non-zero and dumped.
> > +                */
> > +               for (i = 0; i < n; i++, m++) {
> > +                       const struct btf_type *mtype;
> > +                       __u32 moffset;
> > +
> > +                       mtype = btf__type_by_id(d->btf, m->type);
> > +                       moffset = btf_member_bit_offset(t, i);
> > +
> > +                       /* btf_int_bits() does not store member bitfield size;
> > +                        * bitfield size needs to be stored here so int display
> > +                        * of member can retrieve it.
> > +                        */
> > +                       bit_sz = btf_member_bitfield_size(t, i);
> > +                       err = btf_dump_type_data_check_zero(d, mtype, m->type, data + moffset / 8,
> > +                                                           moffset % 8, bit_sz);
> > +                       if (err != ENODATA)
> > +                               return err;
> > +               }
> > +               return -ENODATA;
> > +       }
> > +       case BTF_KIND_ENUM:
> > +               if (btf_dump_get_enum_value(d, t, data, id, &value))
> > +                       return 0;
>
> why not propagating error here?
>
> > +               if (value == 0)
> > +                       return -ENODATA;
> > +               return 0;
> > +       default:
> > +               return 0;
> > +       }
> > +}
> > +
>
> [...]
>
> > +       case BTF_KIND_ARRAY:
> > +               err = btf_dump_array_data(d, t, id, data);
> > +               break;
> > +       case BTF_KIND_STRUCT:
> > +       case BTF_KIND_UNION:
> > +               err = btf_dump_struct_data(d, t, id, data);
> > +               break;
> > +       case BTF_KIND_ENUM:
> > +               /* handle bitfield and int enum values */
> > +               if (bit_sz) {
> > +                       unsigned __int128 print_num;
> > +                       __s64 enum_val;
> > +
> > +                       print_num = btf_dump_bitfield_get_data(d, t, data, bits_offset, bit_sz);
> > +                       enum_val = (__s64)print_num;
> > +                       err = btf_dump_enum_data(d, t, id, &enum_val);
>
> this is broken on big-endian, no? Basically almost always it will be
> printing either 0, -1 or 0xffffffff?..
>
> > +               } else
> > +                       err = btf_dump_enum_data(d, t, id, data);
> > +               break;
> > +       case BTF_KIND_VAR:
> > +               err = btf_dump_var_data(d, t, id, data);
> > +               break;
> > +       case BTF_KIND_DATASEC:
> > +               err = btf_dump_datasec_data(d, t, id, data);
> > +               break;
> > +       default:
> > +               pr_warn("unexpected kind [%u] for id [%u]\n",
> > +                       BTF_INFO_KIND(t->info), id);
> > +               return -EINVAL;
> > +       }
> > +       if (err < 0)
> > +               return err;
> > +       return size;
> > +}
> > +
> > +int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
> > +                            const void *data, size_t data_sz,
> > +                            const struct btf_dump_type_data_opts *opts)
> > +{
> > +       const struct btf_type *t;
> > +       int ret;
> > +
> > +       if (!OPTS_VALID(opts, btf_dump_type_data_opts))
> > +               return libbpf_err(-EINVAL);
> > +
> > +       t = btf__type_by_id(d->btf, id);
> > +       if (!t)
> > +               return libbpf_err(-ENOENT);
> > +
> > +       d->typed_dump = calloc(1, sizeof(struct btf_dump_data));
>
> just realized this doesn't have to be calloc()'ed, it can be on the
> stack zero-initialized variable; feel free to switch in the follow up
> as well
>
> > +       if (!d->typed_dump)
> > +               return libbpf_err(-ENOMEM);
>
> then we won't need to handle this at all
>
> > +
> > +       d->typed_dump->data_end = data + data_sz;
> > +       d->typed_dump->indent_lvl = OPTS_GET(opts, indent_level, 0);
> > +       /* default indent string is a tab */
> > +       if (!opts->indent_str)
> > +               d->typed_dump->indent_str[0] = '\t';
>
> [...]