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btf: Generate correct struct bitfield member types

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For int types, the correct type size will be generated.  For enum types,
if the bit size is not 32, current BTF enum cannot represent it so a
signed int type will be generated.

For the following example:

  $ cat test.c
  enum A { A1, A2, A3 };
  struct t {
    enum A a:3;
    volatile enum A b:4;
  } g;
  $ gcc -c -g -O2 test.c

Without this patch, we will have:

  $ pahole -JV test.o
  [1] ENUM A size=4 vlen=3
        A1 val=0
        A2 val=1
        A3 val=2
  [2] STRUCT t size=4 vlen=2
        a type_id=4 bits_offset=0
        b type_id=6 bits_offset=3
  [3] VOLATILE (anon) type_id=1
  [4] ENUM A size=1 vlen=3
        A1 val=0
        A2 val=1
        A3 val=2
  [5] ENUM A size=1 vlen=3
        A1 val=0
        A2 val=1
        A3 val=2
  [6] VOLATILE (anon) type_id=5
  [7] INT (anon) size=4 bit_offset=0 nr_bits=32 encoding=(none)

There are two issues in the above. The struct "t" member "a" points to
type [4]. But the nr_bits is lost in type [4].

The same for type [5] which is for struct "t" member "b".

Since BTF ENUM type cannot encode nr_bits, this patch fixed the issue by
generating a BTF INT type if the ENUM type number of bits in pahole is
not 32.

With this patch, the incorrect member nr_bits issue is fixed as below:

  $ pahole -JV test.o
  [1] ENUM A size=4 vlen=3
        A1 val=0
        A2 val=1
        A3 val=2
  [2] STRUCT t size=4 vlen=2
        a type_id=4 bits_offset=0
        b type_id=6 bits_offset=3
  [3] VOLATILE (anon) type_id=1
  [4] INT (anon) size=1 bit_offset=0 nr_bits=3 encoding=SIGNED
  [5] INT (anon) size=1 bit_offset=0 nr_bits=4 encoding=SIGNED
  [6] VOLATILE (anon) type_id=5
  [7] INT (anon) size=4 bit_offset=0 nr_bits=32 encoding=(none)

Signed-off-by: Yonghong Song <yhs@fb.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Cc: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Yonghong Song 2018-11-29 15:45:49 -08:00 committed by Arnaldo Carvalho de Melo
parent 70ef8c7f07
commit b18354f64c
2 changed files with 10 additions and 1 deletions
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9
btf_encoder.c
View File

@ -85,6 +85,15 @@ static int32_t enumeration_type__encode(struct btf *btf, struct tag *tag)
struct enumerator *pos;
int32_t type_id;
/* if enumerator bit_size is not 32, generate an int type instead. */
if (etype->size != 32) {
struct base_type bt = {};
bt.bit_size = etype->size;
bt.is_signed = true;
return btf__add_base_type(btf, &bt);
}
type_id = btf__add_enum(btf, etype->namespace.name,
etype->size, etype->nr_members);
if (type_id < 0)

2
libbtf.c
View File

@ -228,7 +228,7 @@ int32_t btf__add_base_type(struct btf *btf, const struct base_type *bt)
{
struct btf_int_type int_type;
struct btf_type *t = &int_type.type;
uint8_t encoding;
uint8_t encoding = 0;
t->name_off = bt->name;
t->info = BTF_INFO_ENCODE(BTF_KIND_INT, 0, 0);