Re: [PATCH] btf: Add support for the floating-point types

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Sat, Mar 06, 2021 at 07:16:08PM -0800, Andrii Nakryiko escreveu:
> On Fri, Mar 5, 2021 at 6:22 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
> >
> > Some BPF programs compiled on s390 fail to load, because s390
> > arch-specific linux headers contain float and double types.
> >
> > Fix as follows:
> >
> > - Make DWARF loader fill base_type.float_type.
> >
> > - libbpf introduced support for the floating-point types in commit
> >   986962fade5, so update the libbpf submodule to that version and use
> >   the new btf__add_float() function in order to emit the floating-point
> >   types when base_type.float_type is set.
> >
> > Example of the resulting entry in the vmlinux BTF:
> >
> >     [7164] FLOAT 'double' size=8
> >
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> 
> [PATCH dwarves] would make it a bit clearer that this is pahole patch.
> 
> But LGTM.

So older versions of bpftool will fail with a .BTF section having this
new float? I thought it would just skip it emitting a warning? Probably
not possible as we don't have the record size encoded in a header,
right?

[acme@five pahole]$ bpftool btf dump file vmlinux | grep -w FLOAT
[acme@five pahole]$ type pahole
pahole is /home/acme/bin/pahole
[acme@five pahole]$ ls -la ~/bin/pahole
lrwxrwxrwx. 1 acme acme 34 Jan 29 11:00 /home/acme/bin/pahole -> /home/acme/git/pahole/build/pahole
[acme@five pahole]$ pahole -J vmlinux
[acme@five pahole]$ bpftool btf dump file vmlinux | grep -w FLOAT | head
Error: failed to load BTF from vmlinux: Invalid argument
[acme@five pahole]$

Perhaps the warning emitted by bpftool should suggest updating the tool
as it found a record type it doesn't know about?

/me goes to update bpftool...

- Arnaldo
 
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> 
> >  btf_loader.c   | 21 +++++++++++++++++++--
> >  dwarf_loader.c | 11 +++++++++++
> >  lib/bpf        |  2 +-
> >  libbtf.c       | 28 +++++++++++++++++++++++++++-
> >  4 files changed, 58 insertions(+), 4 deletions(-)
> >
> > diff --git a/btf_loader.c b/btf_loader.c
> > index ec286f4..7cc39aa 100644
> > --- a/btf_loader.c
> > +++ b/btf_loader.c
> > @@ -160,7 +160,7 @@ static struct variable *variable__new(strings_t name, uint32_t linkage)
> >         return var;
> >  }
> >
> > -static int create_new_base_type(struct btf_elf *btfe, const struct btf_type *tp, uint32_t id)
> > +static int create_new_int_type(struct btf_elf *btfe, const struct btf_type *tp, uint32_t id)
> >  {
> >         uint32_t attrs = btf_int_encoding(tp);
> >         strings_t name = tp->name_off;
> > @@ -175,6 +175,20 @@ static int create_new_base_type(struct btf_elf *btfe, const struct btf_type *tp,
> >         return 0;
> >  }
> >
> > +static int create_new_float_type(struct btf_elf *btfe, const struct btf_type *tp, uint32_t id)
> > +{
> > +       strings_t name = tp->name_off;
> > +       struct base_type *base = base_type__new(name, 0, BT_FP_SINGLE, tp->size * 8);
> > +
> > +       if (base == NULL)
> > +               return -ENOMEM;
> > +
> > +       base->tag.tag = DW_TAG_base_type;
> > +       cu__add_tag_with_id(btfe->priv, &base->tag, id);
> > +
> > +       return 0;
> > +}
> > +
> >  static int create_new_array(struct btf_elf *btfe, const struct btf_type *tp, uint32_t id)
> >  {
> >         struct btf_array *ap = btf_array(tp);
> > @@ -397,7 +411,7 @@ static int btf_elf__load_types(struct btf_elf *btfe)
> >
> >                 switch (type) {
> >                 case BTF_KIND_INT:
> > -                       err = create_new_base_type(btfe, type_ptr, type_index);
> > +                       err = create_new_int_type(btfe, type_ptr, type_index);
> >                         break;
> >                 case BTF_KIND_ARRAY:
> >                         err = create_new_array(btfe, type_ptr, type_index);
> > @@ -442,6 +456,9 @@ static int btf_elf__load_types(struct btf_elf *btfe)
> >                         // BTF_KIND_FUNC corresponding to a defined subprogram.
> >                         err = create_new_function(btfe, type_ptr, type_index);
> >                         break;
> > +               case BTF_KIND_FLOAT:
> > +                       err = create_new_float_type(btfe, type_ptr, type_index);
> > +                       break;
> >                 default:
> >                         fprintf(stderr, "BTF: idx: %d, Unknown kind %d\n", type_index, type);
> >                         fflush(stderr);
> > diff --git a/dwarf_loader.c b/dwarf_loader.c
> > index b73d786..c5e6681 100644
> > --- a/dwarf_loader.c
> > +++ b/dwarf_loader.c
> > @@ -461,6 +461,16 @@ static struct ptr_to_member_type *ptr_to_member_type__new(Dwarf_Die *die,
> >         return ptr;
> >  }
> >
> > +static uint8_t encoding_to_float_type(uint64_t encoding)
> > +{
> > +       switch (encoding) {
> > +       case DW_ATE_complex_float:      return BT_FP_CMPLX;
> > +       case DW_ATE_float:              return BT_FP_SINGLE;
> > +       case DW_ATE_imaginary_float:    return BT_FP_IMGRY;
> > +       default:                        return 0;
> > +       }
> > +}
> > +
> >  static struct base_type *base_type__new(Dwarf_Die *die, struct cu *cu)
> >  {
> >         struct base_type *bt = tag__alloc(cu, sizeof(*bt));
> > @@ -474,6 +484,7 @@ static struct base_type *base_type__new(Dwarf_Die *die, struct cu *cu)
> >                 bt->is_signed = encoding == DW_ATE_signed;
> >                 bt->is_varargs = false;
> >                 bt->name_has_encoding = true;
> > +               bt->float_type = encoding_to_float_type(encoding);
> >         }
> >
> >         return bt;
> > diff --git a/lib/bpf b/lib/bpf
> > index 5af3d86..986962f 160000
> > --- a/lib/bpf
> > +++ b/lib/bpf
> > @@ -1 +1 @@
> > -Subproject commit 5af3d86b5a2c5fecdc3ab83822d083edd32b4396
> > +Subproject commit 986962fade5dfa89c2890f3854eb040d2a64ab38
> > diff --git a/libbtf.c b/libbtf.c
> > index 9f76283..ccd9f90 100644
> > --- a/libbtf.c
> > +++ b/libbtf.c
> > @@ -227,6 +227,7 @@ static const char * const btf_kind_str[NR_BTF_KINDS] = {
> >         [BTF_KIND_FUNC_PROTO]   = "FUNC_PROTO",
> >         [BTF_KIND_VAR]          = "VAR",
> >         [BTF_KIND_DATASEC]      = "DATASEC",
> > +       [BTF_KIND_FLOAT]        = "FLOAT",
> >  };
> >
> >  static const char *btf_elf__printable_name(const struct btf_elf *btfe, uint32_t offset)
> > @@ -367,6 +368,27 @@ static void btf_log_func_param(const struct btf_elf *btfe,
> >         }
> >  }
> >
> > +static int32_t btf_elf__add_float_type(struct btf_elf *btfe,
> > +                                      const struct base_type *bt,
> > +                                      const char *name)
> > +{
> > +       int32_t id;
> > +
> > +       id = btf__add_float(btfe->btf, name, BITS_ROUNDUP_BYTES(bt->bit_size));
> > +       if (id < 0) {
> > +               btf_elf__log_err(btfe, BTF_KIND_FLOAT, name, true, "Error emitting BTF type");
> > +       } else {
> > +               const struct btf_type *t;
> > +
> > +               t = btf__type_by_id(btfe->btf, id);
> > +               btf_elf__log_type(btfe, t, false, true,
> > +                                 "size=%u nr_bits=%u",
> > +                                 t->size, bt->bit_size);
> > +       }
> > +
> > +       return id;
> > +}
> > +
> >  int32_t btf_elf__add_base_type(struct btf_elf *btfe, const struct base_type *bt,
> >                                const char *name)
> >  {
> > @@ -380,7 +402,11 @@ int32_t btf_elf__add_base_type(struct btf_elf *btfe, const struct base_type *bt,
> >         } else if (bt->is_bool) {
> >                 encoding = BTF_INT_BOOL;
> >         } else if (bt->float_type) {
> > -               fprintf(stderr, "float_type is not supported\n");
> > +               if (bt->float_type == BT_FP_SINGLE ||
> > +                   bt->float_type == BT_FP_DOUBLE ||
> > +                   bt->float_type == BT_FP_LDBL)
> > +                       return btf_elf__add_float_type(btfe, bt, name);
> > +               fprintf(stderr, "Complex, interval and imaginary float types are not supported\n");
> >                 return -1;
> >         }
> >
> > --
> > 2.29.2
> >

-- 

- Arnaldo